I have not actually bought those kind of goods in St. Maarten but I have in St. Thomas, many times, and providing you say that you want them to work in the UK area they will be fully au fait with your needs. I have never had a problem with goods and the guarantee with the manufacturer is there particularly if you register it with them on your return. Get some copies of the items you want together with the spec and price and take it with you and if it is a well known manufacturer and the price really is good then buy it......Nei.
Do check all the prices here first before you go...we made the mistake of thinking we got a bargain camcorder in the Canaries a few years ago only to discover that Jessops had it for £50 less here!!! However the guarantee was valid when it eventually went wrong and we had it repaired free of charge!
Walk down the quay from the ship and turn left down the main street, first camera/laptop shop on the right had a good deal on Nikon which I bought but also did Canon too
( our ships staff recommended ) We were put off by the shops on the left - different names but same ownership - the staff ( owners ? ) went out of the back entrances to meet us in the next shop(s) ! As always familiarise yourself with numbers and prices beforehand - Sometimes numbers are different ( Canon Compacts for example ) BTW not much saving on the SONY my pal bought - hope this help.