for the ultimate in reliability and dependability, install a 60 watt solar panel with a quality controller. it will work when you are away from shore power, so you are never without unless you have equipment failure...
once installed, you should reasonably expect 20+ years from it with very little maintenance.... and with the quality controller it will keep the battery properly charged, unlike many of the cheap float chargers that is on the market....
I have installed solar on my boat after using the charger for too long, and I have become a proponent for it due to the simplicity and dependability of the system...
BUT, it depends on your use. if you are only at the boat occasionally and you dont consume much power when you are there, then a smaller panel would work, but a 5watt panel will barely keep up the natural power loss of a group 27 battery. a group 24 battery will be a bit better, but will be extremely slow to bring the battery to a full charge of 13.6 volts.
a standard music radio without an amp, and playing softly consumes 1.5 amps per hour... with the cd player working, about 2.5 amps.... if you crack the volume up, the consumption can go up to about 5amps. (unless you have LED lights, the incandescent bulbs will consume 1-2 amps per hour)
if you use the radio for 6 hours in a day, you have consumed a minimum of 9amps. in the evening with a light on for a few hours it will quickly grow to 15 amps of use.
so to bring the battery back to full charge in a reasonable amount of time you need a reasonably sized system.
5 watts = 0.384 of a volt, less than almost all trickle chargers you will find.... and that is when the conditions are perfect.
a 5watt panel works fine as a maintainer for small batteries like an ATV battery, but as the battery size increases, so does the amperage and natural power loss (self discharge of 2% to 5% per month) so you need a bigger charger to maintain it properly.
AND...when you are dealing with solar power, expect only about 50% efficiency from the rated output of the system over a period of time.... some days will be better and some days worse.
a 60watt system will give you a dependable 2 amp charge minimum and will probably keep ahead of your highest demands for now, but if you do some research on actual installations that people have done, you may come to understand it better.
its almost an "install it and forget it" system, but you will still need to service your battery regularly.
of all the modifications and add-ons i have installed on my boat, the solar power system is by far the most useful.... it has taken the time, trouble and worry out of having a dead battery and the PITA of charging it back up....