These links are not definitive or exhaustive. Just the ones I use most.
Model Boat Mayhem, - Probably the best place to start for everything 'model boat.' UK based forum where even arduino is accepted. (I could direct you to 'establishment' forums in very well known places where arduino is not well accepted, but I am not saying anything more. (email me))
Microgyros: Mike's site is off the air right now. He makes the best speed controllers, self levelling devices and ballast control systems on the web. (Author's biased opinion) The link is to a discussion group.
Hobby King, and Ebay for nearly everything realted to RC.
Cornwall Boats for plans and they have a great range of parts.
Metro Hobbies, My local Victorian hobby store
Float a Boat - My local boating hobby store
The SubCommittee - The place for every question
Dive in to Submarines - A smaller facebook site that has expertise dripping from it. Thanks Andy!
Sub Driver - David Merriman's sub forum and site
RC Submarines - Bob Martin's site. The place to go to buy a custom build sub. His production standards are fantastic.
R&R Submarines - The home of Ronald Perrot. Ron is the grandfather of British submarining. His products are the very best.
Hobby King RC groups - Submarine forum
Arduino - The home page for all things arduino
All about circuits - has a terrific section on arduino.
Radio Control hub - This is a fantastic new site for people into RC who want to get into Arduino. A couple of my vids feature here, but I feel this is a privileged as the site has fantastic links. New to Arduino, start here.
Sparkfun - As the name suggests this site is about ideas and Arduino hardware , of all shapes and sizes. A cornucopia of great gadgets and tutorials. My 'helping hands' come from here as do several other items I have utilised.
Adafruit - got to be seen to be believed. Lots of out there stuff and lots of tutorials
Arduino and Boats
Robo Sail - A magnificent site with great ideas. Even young people getting into building stuff
Surrey park Model Boat Club - my home base