Can you mount it to the dock?
How quickly do you need to charge?
How many batteries to you have?
I built a solar powered stereo system for my kids playground. There is a little info in
this thread.
I used this
$60 30 watt solar panel http://www.amazon.com/gp/product/B00COBQR14/
And this
$20 solar charge controller http://www.amazon.com/gp/product/B00752TRNW
The solar power is stored in a marine battery.
Solar power is highly dependent on where you live. 4 hours of peak production seems to be the norm for central Texas. So you can expect the 30 watt panel to produce 120 watts hours (10 amp hours) per day. The charge would peak at a rate of 2.5 amps. So this is a trickle charger. Your battery likely has around a 100 amp hour capacity. You will damage it if you discharge below 50% so you are unlikely to run it that low on any outing. So the 30 watt panel should give you a full recharge from the maximum reasonable discharge over 5 days.
Ideally I would mount the panel to the dock with the charge controller on the back of the panel. Then I would run 14 gauge wire for up to 15 feet for 12 gauge up to 25 feet and use anderson power poles with dust covers as a quick connector.
The kids solar panel has kept their battery charged. They have always had power to play their music. Somedays they leave it running all day. So it is working well for us.