Anna/Mark
Great work as usual Mark. Unfortunately these brick walls appear all to frequently. Mr Watlen's illustrious naval career is certainly proving elusive.
With that in mind, I thought I'd have one more look and decided to take the high road to Scotland and see what they might have archived.
He turned up in the National Library of Scotland website. A couple of references to his sheet music compositions that we already know about were there but then a long transcript listing Scottish business's and their owners and other individuals for the period came up. It looks as though it has been made up from various directories and marriage records etc. and was a little disjointed in style. However John Watlen's notes in it were as follows:
John Watlen - music warehouse Edinburgh
13 North Bridge 1793-94
North Bridge 1795
34 North Bridge 1796-97
music master Hermitage, Abbey Hill 1799
No number 1794
Married Mary daughter of late Archibald Megga(e)t writer 4 March 1797
John Watling(?) musicseller North Bridge admitted to Sanctuary for debt at Holyroodhouse 25 June 1798
Edin.Dir: Edin Marr: Cadell
With this little bit of family history, I thought I would try that route and searched his name and London address in Leicester Sq.
This led to a page belonging to a private family tree
http://www.debenham.org.uk/william/John Watlen.php . The site itself is dedicated to research in Debenham & Debnam Family History.
John Watlen's entry is in the form of a Timeline and is easy to follow. It confirms a lot of what we have already discovered and provides some further information about his later life, although it doesn't focus that much on his actual composing or teaching career and unfortunately for us, does not mention his naval career at all.
The Timeline has him being born in 1764, this is an estimation calculated from his age of 69 recorded in St Anne's Church , Westminster parish register, on his death on 2 October 1833. From 1764 to his taking up employment with Corrie & Co around 1788 there is no information recorded about him at all. But plenty of time of course in that period to have had some sort of career at sea. A pity we don't know more of his parents background or where he was born.
So one all with the brick wall's Mark and like you, I think I have exhausted my research ideas too, certainly for now.
Phil