Hi Ruairi,
i dont know about the construction of these boats, but you need to find out how the underfloor structure is put together. This is not always easy to find out though.
i.e. is there any foam.
However, I would suspect that there is very little foam under the floor and it is more about cavity buoyancy.
The best way to check if there is water trapped is to get her out of the water on a trailer (turn the sump pump off before you do so it doesnt pump out while recovering) and wiggle her around (ensure to account for the fuel sloshing around). Tip up the bow with the jocky wheel and see if that sends water to the rear drain/sump pump.
If it does, you might need to do some calcs about expected weight and get her weighed (checking for water logging)
if it is catching rainwater, you might want to check the weight distribution in the boat. i.e. where is the battery and the fuel tank? these things have a huge impact on how the boat sits without passengers and could be causing it to sit incorrectly and prevent the water from draining, causing the viscous cycle.