There are several possibilities to your problem.
1, Remove the end cover off your Jabsco seawater cooling pump, check the vanes are all intact, if some are broken, drain down any water then get a wet & dry vacuum cleaner, place the nozzle UP the pipe towards the heat exchanger to suck out any broken impeller vanes or debris, then, replace the new impeller using washing up liquid as a lubricant (it also lubricates the impellers during the first dry start up). Run the engine and check for leaks.
2nd, using a proprietary radiator flushing liquid clean the cooling system as per directions, if necessary do it twice, when refilling after cleaning the cooling system fill with an anti-freeze mixture.
Even in hot climates, the AF mixture contains both a lubricant for your recirculating water pump and a cleaner to keep your water galleries clean to aid dissipation of heat via radiation and I highly recommend keeping it in all year round.
3rd, are you over propped ?
I hope this helps you solve your problems BUT, fix the problem and under NO circumstances let the engine overheat