1. upgrade to 50A 120/250 service; or
2. add a third 30A inlet/line/breaker/etc.
The first is by far the more preferable, though not cheap
That would give you two 50A @ 120V lines from the boat inlet (giving you a total of 100A compared to the existing 60A).
The 50A inlets are the same size as 30A, so would fit in the existing hole.
You'd need to upgrade the wiring
in the boat to AWG 6 from the inlet to the breaker & panel(s).
Suggest you contact a local qualified marine
electrician before proceeding.