Just carry a jumper and don't worry about it. You should be able to last a long time though without any amps.
We just bought a simple/cheap, am/fm/cd/mp3 player and 2 small speakers for the boat. My question is i'm trying to avoid 2 batteries. Were running a red top optima given it is in good shape how long should we be able to float with the radio on before having to worry about starting?
Just carry a jumper and don't worry about it. You should be able to last a long time though without any amps.
Without amps, and not cranking the volume (which with a cheap deck and just two speakers is unlikely) you should be good all day.
Get a jumperbox carryon and you can just hook that up to start if you have any problems....then go for a boat ride faster than idle to charge them back up.