Not knowing what a 21 Reynolds is I jumped to the conclusion that it was an outboard
center console or something. Sorry, I should have figured it out from your user name.
Get at least a Group 24 AGM
battery. This will have about 60-75 amp hours of capacity and will run your stereo and lights all day (and night). Your 6 watt solar
panel should work ok for recharging as long as you don't use the boat for a week in between. But the solar panel will be a bit marginal.
put out approximately 1/3 of their wattage rating in amp hours during a normal, mostly full sun day. That is only 2 amp hours. For the 6 days in between this will recharge only 12 amphours. But it might work if you don't use the boat every weekend. With a Group 24 you will have enough capacity to cover several weeks of weekend use.