I'm in the same position; same size boat and probably same size batteries - Group 24's w/ ~75 amp-hours each. My research seems to point that a 5w panel is enough to keep the batteries up to level with standard discharge (1%/day), but not enough to make up for losses during usage. I'm been thinking about springing for a 30W panel (~$250) and a pair of low-cost controllers.
The only decent dual battery controller I've seen seems to cost $150+ from the lowest cost sellers and looks like more controller than necessary.
One controller is theoretically ok for the bank, but if you permanently wire them in, you need to add a switch to break the connection as the batteries would be wired in parallel. Lower cost controllers can be had online for $20-30.
Smaller solar panels don't seem like they're going to be sufficient to compensate for actual usage. Estimating somewhat conservatively, a 30W panel can put out around 10 amp-hours per day (dividing the wattage by 15 to get to amp-hours and then multiplying by an optimistic 5 hours/day of power) and a 5W only 1.65. Putting that into perspective, my Raymarine A65 at full brightness, an average VHF or stereo are all rated at around 1 amp per hour. Multiply by 5-6 hours of sailing and there isn't much left to charge the batteries for lights!
A random thing I found out is that if I moved to a Group 27 battery (2" wider than a Grp 24), the capacity goes up by 30-40%, depending on brand. I needed new batteries and swapped in wet-cell 24s this past weekend. I wish I had thought about this in the spring when I had more time and I would have put in a new battery box to accommodate the larger batteries.. I paid $90 for each and could have bought Group 27s for $100 each, both quality batteries from a reputable generator shop. The 27s are definitely worth the extra $10 if you can squeeze them in.
A great reference is Don Casey's article:
Installing a Solar Panel to Maintain Batteries by Don Casey