question

AGM batteries do deteriorate with age and use, especially heavy use. Impossible to say with certainty how good the house bank is.

One reason for keeping house and starter batteries separate is to prevent house loads from depleting the starting system. Another is that direct connections to the alternator will shorten the house bank life due to overcharging.

Might be better to get ready to replace the starter battery.

@codefoster Hello. I am not sure what sort of boat you have or what size engine but I know from my experience other factors can come into play. A boat usually has a start and house battery isolator. In my case this isolator is notoriously bad for intermittent voltage drop. Mine stay dormant for years and occasionally need to be vigorously exercised or they tend to develop a high resistance joint internally. Connections from the start battery to the starter motor need to be checked. In my case the engine is a small Yanmar so should be easy enough to crank. In your case your starter battery may struggle but you have paralleled it with a comparitively massive house bank. Even if the house bank is a bit old, it should still really crank that boat motor quite easilly.

Yes the start battery may be reaching the end of life but the paralleling with the house bank should make a big difference to the cranking ability. If that is not the case, it may indicate that the connections through the isolation switch or other connections may be worth considering as a cause.