In general terms you should try and vaguely match Watts, so if the ouput of your hi fi is 500 Watts you don't want to put 50 Watt speakers and vice-versa. The output of your unit is 50 Watts so I would think anything up to a 100 Watt rated speaker would be fine. It can then handle any peaks of sound with ease.
Ohms are the other thing. An Ohm is a unit of electrical resistance, quite boring. However it's worth double checking that any speaker for your boat is 4 ohms - to match your amplifier which will, undoubtedly, be 4 ohm. It's highly unlikely that you could buy a marine speaker that wasn't 4 ohms, but worth checking all the same....