- Do you just want to be tracked or have 2 way msg'ing capability?
The Nuvi 350 and OT is great for the latter, the cheaper OT1+ is great for the former.
Being on a boat, (You didn't mention sail or power) you might need some power if the antenna is just above sea-level (on the deck). If it's a sailboat, then hoisting an antenna atop the mast is the way to do it and go QRP (an HT running off the 12 DC system) The 450 ohm ladder line J-pole antenna ( homemade ) works great from high trees, masts, etc.
So to answer the question, more info is needed.
KA1GJU -2, -3, -6, -7, -8, -9