With your existing system, it looks like you only have two options. Either leave powered on float or disconnect completely and have someone attend the boat every 3 months and top them up.
The reason they only last 6 months is that they contain internally embedded electronics which consume some small amount of power. All LFP batteries with internal BMS will have this same problem. The cells themselves last much longer (years) without needing topped up, but any batteries with internal BMS will always drain down slowly.
As to which option is preferable, my personal choice would be to have them topped up periodically. LFP do not like to be floated at high states of charge for long periods as it does negatively impact their life.
In my system I have 115Ah of Lead Acid in parallel with my LFP cells, so my wintering strategy involves dropping the LFP cells to about 60% SOC and disconnecting them completely from the system and the (external) BMS. Unfortunately this is not possible if you have batteries with an internal BMS.
Sorry, not much help!