We looked at how Halley came to be given command of a Royal Navy ship during his first voyage and I said we’d examine what experience he had to justify that commission during his second: this we’ll do now in a two-part special.
In fact Halley had minimal maritime experience – although perhaps rather more than your average seventeenth-century natural philosopher – but the experience he did have was of three types: as a passenger, as a surveyor, and as a diver.
Halley was a passenger on three voyages as a young man, the first when he abandoned his degree in 1676 to sail to St Helena to map the southern stars. The two-way voyage on an East Indiaman would have taken about 5 months, and as Halley seems to have had a life-long interest in maritime matters, it seems reasonable to assume he took the opportunity to observe how the ship was sailed, how the crew operated, and to pick up a smattering of nautical terminology.
One year after his return from St Helena, he was at sea again en route to visit the great astronomer, Johannes Hevelius, at Dantzick (Gdańsk) on the Baltic coast, and at the end of the following year, 1680, he crossed from Dover to Calais (and was apparently seasick) to begin his Grand Tour, returning to England from Holland in January 1682.  Again, it seems fair to expect that he paid at least some attention to the handling of the ships.
At the end of the decade, Halley progressed from maritime passenger to coastal surveyor, although his work as a surveyor is rather obscure, being largely inferred from his presentation of two charts to the Royal Society, the first of which appeared in the minutes for 3 July 1689:
Halley produced his Sea-draught of the Mouth of the River of Thames, wherein he saith, that He hath corrected severall very great, and considerable faults in all our Sea-Carts [sic] hitherto published. 
It isn’t known for sure when he undertook his survey of the Thames approaches but Robert Hooke recorded two entries in his diary – “Hally a Sayling” on 22 March 1689, and “Hally Returnd” on 3 April – which may relate to his surveying, although the previous summer Halley had made several references to towns near the Thames estuary which may indicate that he had been surveying in the area at that time. 
His second survey, “of the West coast of Sussex between Selsey and Arundell”, was presented to the Society on 15 November 1693,  and this time we have rather more idea about the circumstances behind its production because this chart was a by-product of his diving activities.
Halley first wrote about diving in a paper of 6 March 1689, and his interest was probably prompted by the recent success of Captain William Phips’s diving operation in the West Indies, where Phips recovered a spectacular haul of treasure from a Spanish wreck and sparked a tsunami of projects connected with diving and salvage.  Halley proposed a mobile diving bell built on four wheels, and while he didn’t build that particular bell, he did build another as part of his salvage work on the wreck of the Guynie frigate.
The Guynie was owned by the Royal African Company (RAC) and in early 1691 she returned from Africa to English waters, having collected “Bees Wax & Elephants Teeth” from Gambia and “Elephants Teeth & Redwood” from “Sereleon” (Sierra Leone).  On 23 February her commander, William Chantrell, wrote to the RAC from Falmouth requesting a convoy to accompany him back to London. The RAC instructed three of their most senior officers to “waite on the Lords of the Admiralty to gett a Man of Warr to Convoy the Guynie Friggott up from Falmouth”, which might seem a little excessive for beeswax and ivory, but the Guynie was carrying something much more valuable on board. 
Captain Chantrell had undertaken to deliver a large quantity of gold on behalf of the Portuguese in Africa, and it was on this account that a convoy ship had been requested.  The Guynie sailed from Falmouth on 26 March, but on 4 April Captain Chantrell wrote from Chichester to advise the RAC that the ship had foundered.  It isn’t clear what happened to the ship, but there’s no mention of any lost men and the gold seems to have been saved, as on 8 April the RAC ordered that “a Guard of Tenn Soldjers & an Officer be sent from hence to fetch the Gold (saved out of that Shipp) from Chichester”. 
The ivory, however, went down with the ship, and it’s in this regard that Halley now appears in the affair. The Deputy Governor of the RAC was Royal Society fellow, Abraham Hill, and it may have been he who brought Halley in to try to salvage the elephants’ tusks. Halley’s initial plan was certainly ambitious as on 13 May 1691 the Royal Society minutes record that “Halley shewed the Method he intended to use in raising the Ship”, but this plan was evidently modified as on 12 August he was relating “the Success of his Experiments of going under water in his diving bell”.
Halley believed his diving bell had limitless possibilities, and on 15 September a government warrant was issued to prepare a bill granting Halley and three partners “sole use of their invention of a new engine never yet known”, and Letters Patent were issued on 15 October. On 17 November, the four patent-holders formed a joint-stock company with two other men, John Carter and Thomas Jett, who was a friend of Halley’s. 
Halley’s signature on the agreement forming the joint-stock company (© National Archives (£), C 111/192)
Thomas Jett, incidentally, is interesting because he is the “Jed” in Hooke’s Diary entry for 24 March 1693, “Hally & Jed Spys”, which offers a hint (there are one or two others) that Halley may have undertaken coastal surveys on behalf of the government. 
Halley and Jett appear periodically in the RAC minutes until about spring 1694, though their level of success in salvaging the tusks is unclear – and by that time Halley had become involved in the scheme with Benjamin Middleton to obtain a ship to sail around the world, which, when the scaled-down version of the project finally got underway in late 1698, would see Halley drawing on all his modest maritime experience to serve as the Paramore‘s commander. 
In part 2 of this post about Halley’s nautical experience, we’ll take a much closer look at his diving bell – and at my favourite thing about him!
 Hooke to Halley, 1 March 1681, Royal Society, EL/H3/62; Hooke, Diary, 24 Jan 1682, London Metropolitan Archives.
 Royal Society, Journal Book Original, JBO/8 p 268.
 Hooke, Diary, 22 March 1689 and 3 April 1689, British Library, Sloane MS 4024; references to towns in RS, JBO/8.
 RS, JBO/9 p 141.
 For a fair copy of Halley’s paper, see RS, RBO/7/24.
 RAC Instructions to Captains, The National Archives, T 70/61 ff87r-88v.
 RAC Minutes, TNA, T 70/83.
 RAC Black Book, TNA, T 70/1433 p 104.
 TNA, T 70/83 f8r.
 CSP, D, 1690-91; TNA C111/192.
 BL, Sloane MS 4024. Many thanks to Hooke’s editor Dr Felicity Henderson for confirming this entry as it has hitherto appeared mistranscribed.
 I’ve read suggestions that Halley and Jett only recovered one tusk, but that is based on a mistranscription of the phrase “the elephants teeth”, where the e’s in teeth are akin to the Greek letter theta (ϑ) and have been misread as ‘oo’. This style of ‘e’ was common in the 17th century and Halley himself used it. He later told Hans Sloane that his attempt to recover the ivory had been frustrated by the wreck quickly becoming covered with “Sand and Oase”.