You want fewer batteries (four instead of eight) and to recharge them (properly) more often. Can you use solar panels with the houseboat? Portable solar if necessary?
What model and size in watts is the "little red Honda"? You need enough watts to run the battery charger--do not use the DC output of the Honda
why only a few amps charging from shore power? Why not use the charger in your inverter/charger?
First read this about the care and feeding of batteries, including how to charge AGMs:
https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdfWhat engine power for the houseboat? What batteries if any for that and how recharged? Any way to link that with the "house" 12v system? ( I don't know how houseboats are set up compared with RVs)