I'm quite surprised that you have a Hydronic 10 installed on a boat of that size. If you recall, I mentioned that we have a Mikuni MX40 heater on our boat, which is also 60' in length. Although its output is less than half of the Hydronic 10, it supplies enough energy to three double radiators and the calorifier, to heat the boat and the domestic hot water sufficiently.
It may well be that your heater is over-specified and it heats up too quickly, causing it to idle too often during its operational cycle. This could be why you're having annual problems with reliability. We once had neighbours who owned a 57' x 10' widebeam that had a Hydronic 10 installed. They too had similar problems, until they replaced it with a 5kw alternative. These heaters have to be worked hard, in order that they don't 'coke-up' inside the combustion chamber and around the flame detection device or glow plug.
With regard to the capacity of the pumps, I wouldn't think either of those would have trouble with your system. Remember that they're not actually having to pump a head of pressure, but simply recirculating an otherwise static volume of coolant around a level with a very small gradient. If you were to consider a gravity fed central heating system, where there's no pump installed, the water circulates efficiently simply by its difference in weight in relation to heat absorbed.
You also mention that the heater struggles to warm rad 5 in the single bedroom, have you tried balancing the system using the lock-shield valves on the rads. By metering the coolant through rads 1 to 4 and leaving rad 5 almost fully open, you may find that it heats up more efficiently. It's certainly worth a try!
Mike