You'll need a solar panel array matched to the size of your boat's available square footage and to the bank size. There are dozens of Youtube videos about that, so I'll just assume you've got some kind of array bringing 50 amps on average into a single input.
You'd want to position all of the following in an enclosed compartment that has good airflow, and consider including temperature regulation/monitoring equipment in that compartment too. This units can get hot. Only #3 below is designed for marine use, though even that one is just water resistant, not waterproof.
1) MPPT
You plug that input into a 48V MPPT. It's labelled 48V because that's what comes out of the output. Keep in mind that if your panel array is below 48V, you'll need a boost MPPT (it does step-up conversion), whereas most 48V MPPTs are typically designed to receive input from higher-voltage arrays, so they'll take in more than 48V and step it down to 48V. For example, I've never used this one, but EPEver is a popular MPPT brand who makes a 48V model (
https://amzn.to/3L8tTvV).
2) Balancer/Equalizer
You'll also want a battery balancer/equalizer. It's basically a BMS but it's external to the pack and it typically works on more than just lithium batteries whereas BMSes are often designed for a single type of lithium. In your case, you just want to make sure the balancer/equalizer supports lead acid. There's a difference between active balancing (taking amp hours from one pack and giving it to another) versus passive (bleeding amp hours off the highest pack). Active is better, but typically more expensive. Also, the price will tend to reflect the current capability. For example, I've never used this equalizer from Kazava, but it supports up to 4 lead acid batteries and it claims active balancing capability of up to 10 amps (
https://amzn.to/41C100C).
3) Charger
And of course, if you happen to also want to charge up the bank via shore/home power at any point, the popular choice is the Noco Genius. They make a 4-bank charger which will charge each pack individually at 10 amps apiece (
https://amzn.to/3UOWTw3).
There's no telling if your panels alone will always top you off without extensive experimentation. The bigger array you install, the less likely you'll ever need to plug in, but who knows? If it's overcast and you want to get out two days in a row, it's quite possible that any array you install will not achieve 100% by the following day.
If you want to charge via solar only, you can get 1 & 2 and skip #3. But if you want to add #3 for convenience, you can technically skip #2 as long as you don't go too long without using #3. #3 will balance the packs similarly to #2, negating the need for #2 any time you use it, but if you intend to charge via solar a dozen times in a row or more before plugging in at shore/home, then you should include #2 to balance in real-time.
