As house bank
and engine starters... Been using four (4) wet cell
Group 31 Deep Cell Marine Batts from "BatteryPlus" store for years. East Penn built, high quality, inexpensive, no problems, easy to maintain, charge quickly, and... collectively supply plenty of power. Also have a gen set
Group 27 starter batt... and... a completely independent
Group 27 deep-cell/starter combo batt that's in separate batt box. I keep it 100% charged in wait for if all else fails; basically it acts as a fool-proof safety feature!
We're very efficient on how elect power is used aboard boat. At anchor for long as we want gen set usually runs about .75 hr in morn and .75 hr in eve. That keeps all batts charged, AC refrigerator/freezer cool, food cooked, coffee perked, water hot, 2 computers/2 cell-phones charged, recharged batts on low level cabin lights... and what ever else we need... like occasionally portable power tool batts charged. Oh, and that includes our 5 mile walkie talkies.
Cost for whole set up from BatteriesPlus was under $650... inc all batts, 2 battery testers, a trickle charger for independent safety batt and its seperate batt box. Over 5 years now and it all is still going strong!
BTW...
I never leave my boat plugged into dock when away from boat for any extended periods (i.e. one 24 hour day or longer). Always keeping boat completely isolated from any contact with dock and making sure that batts aboard are isolated from boat via Perko mains. This makes certain boat gets little as possible corrosion/galvanization/electrolysis to metal portions; I do keep a float switched bilge pump direct-wired to house bank. The only down side to this is that "if" boat suddenly sprung a consistent and sizable leak the batts would eventually go dead, direct wired pump would go quiet... sinking boat sounds such as Blub, Blub, Blub might be heard if no person on dock took actions before boat sunk - That's why I carry insurance!
In my experience of many decades... Wet cell batts do NOT need to constantly be kept to 100% charge. Especially deep cell type. Long as bats are not left long enough that they go below 50% +/- charge, and they have plenty electrolyte in each cell, they are just fine to let stand. Then when visiting boat - turn on charger and bring batts up to 100%. I have gone up to 4 months between visits and found that batts remained at 60 to 70 percent charged. Upon visit I always make certain all batts are at 100%. I'd guess that even six or a little more months would not be way too long between recharge. Because batts do loose charge even while sitting isolated and dormant... there is a limited period of time between which recharge should be accommodated.
Matter of fact, I believe the constant hookup to chargers, therefore non stop charging and recharging of batts (at least wet cell type - which I use) reduces their overall life span.
For even more safety value I've solar panel on front of fly bridge that trickle charges to gen set batt when out and about. That way if all else failed I could restart batt charger. And, as last step caution, I keep sturdy jumper cables aboard too.