IMHO - any stereo less then 50,000 watts is worthless
Seriously - the real difference in a marine radio is how the board gets treated and O ring seals.
Salt air often finds a way in but they do hold up better then a regular radio.
The cases with a O ring seal help too.
One tip - be sure to play it / tune it at least once a month - even in winter.
Helps keep everything in working order.
Using tinned wired for speakers goes a long way too.