There has been no mention of the seacocks. If some previous owner has misguidedly fitted gate-valves, rather than the ball-valves that are normally used, this may be at the root of your problem. If an old gate-valve has failed in the closed, or partially closed position, it will not allow enough
water through to efficiently flush the bowl. Gate valves can be distinguished by the fact that they are operated by a hand-wheel, rather than a lever that turns through 90 degrees. If you do find that you have gate valves, you will have to
haul out your
boat and replace them with ball valves (search "ISO 9093-1:2018", for the best type to use on recreational craft).