The day I swung by to check on his boat, it was about 10:15 am and a hazy but overcast day. This was his battery voltage at 10:15 in the morning.... D'oh.. The battery could not pass a CCA test. When I checked in with the owner he had not used the boat for about 6 days.
The battery was replaced, again, but this time a small & inexpensive PWM controller was installed. I used a Morningstar SunKeeper SK-6-12V, for about $38.00, and problem solved. The cost of this controller was less than half what he had been paying for each battery his unregulated solar panel had destroyed. The SK-6 is PWM with bulk, absorption & float plus it has the ability to add an optional temp sensor. All this for under $40.00.
If your boat is going to be left unattended, for periods of time, with no loads, then you will want a solar controller especially if your panel is actually capable of any meaningful charging to the battery.
A Morningstar SunKeeper SK-6 will cost you about $38.00 - $42.00 or about 1/3 to 1/2 the cost of a new battery. Morningstar builds very good quality controllers and stands behind them.
If the cost of a drive-through dinner at McDonald's, for a family of four, is too much to stomach there are certainly much cheaper Chinese PWM controllers out there. They start at around $10.00.
Unfortunately I have seen too many of these fail and one even catch fire so I don't have a cheap Chinese controller, with a long enough proven track record, to recommend at this time.
If you want to go cheap PLEASE READ THE SPECIFICATIONS! Why? The no-name Chinese brands are famous for selling you something on the sticker then not putting what is on the sticker in the box. What do I mean by that? How about controllers marketed and sold as MPPT that are nothing more than PWM? Seen it numerous times. Even worse I have found Chinese labeled MPPT controllers that are nothing more than simple ON/OFF or what are called "shunting" regulators. ON/OFF regulation is Dinosaur era regulation technology. There are also a metric $hit-ton of Chinese marketed PWM controllers that are not PWM at all, but rather simple ON/OFF/SHUNTING controllers.
HINT: Gibberish Chinglish Specifications = No Buy
You'll want a true PWM controller with specified absorption and float voltages eg: Absorption 14.4V - 14.8V & Float 13.2V - 13.8V etc..
Below is an actual specification for an $18.00 Chinese "PWM controller", a customer of mine purchased off eBay that was really ON/OFF. No, I am not kidding..
Power Regulate: PWM
Rated Voltage: DC12V
DC Current: 5A
Voltage of Solar Panels: 60W (5A,12v)
Charge-Stop Voltage: 14.2V
Charge-On Voltage: 13.2V
PWM controllers do not "stop charging" at 14.2V then allow the battery voltage to decay to an ON voltage 13.2V and return to the "charge-stop" point > repeat>repeat>repeat... This is a prime example of an ON/OFF or shunting controller being sold as a PWM. A complete and utter rip-off.
A controller that can keep your batteries safe, when you're not there, is a very inexpensive investment compared to ruined batteries. Stay safe out there...!