I took the maximum rainfall figures from the met office site and used 14sq m as a "catchment area"
I think there is a slight exagerration in how much water is filling the boat as to fill to the top would take around 12 of water to fall in 24hrs and the catchment area is the size of the bit being filled so it is still a full 12 inches rather than being concentrated into a small area than the "catchment area"
Taking the rough area of the boat it would take around 3000 litres to fill a boat that size give or take a few litres for the exact sizes.
If this much water fell in the two days you happened to have the boat not pumped I would say either you are the worlds unluckiest person, Wales is permanently under water or its ever so slightly exagerrated........
1 inch of rain falling in an area of 1 sq m is still 1 inch deep if it falls over 10 sq m.