Nasher, my last trailer had home made hydraulic drum brakes. Seizing on was not a problem but the cast iron drum corrosion always wore out the linings quickly and, originally, the cylinder pistons corroded easily. I made brass pistons, machined the recesses for the rubber boots a little larger diameter than standard and incorporated an O ring outwards of the hydraulic seal. Filled every space with brake grease and that lot sorted the problem.
I used a tandem master cylinder from a Vauxhall Cavalier which was connected to a hand brake lever which was pushed by the hitch. No problems at all with that. It had a waterproof front boot on it. The hand brake was applied by connecting the winch rope to the top of the hand brake lever. Although, I can count on one hand the number of times I needed it.
I reckon standard cast iron discs and the usual chromed caliper piston would last, unseized, about 5 minutes after saltwater immersion. Brake pads normally contain iron as the abrasive and in normal use they cause corroding of the disc so, in salt water, I'd bet they rust on in no time. I believe there are moves afoot to remove the iron as an abrasive in the future.
I have made caliper pistons from both brass and stainless and both worked fine in normal road use. You could probably make some nice bronze ones for the sea.
I'm just entering the world of 'real' trailer brakes and there design amazes me...which Bozo designed them. As an aid to convienient reversing, they don't work if the trailer is moving backwards
... and they are leading and trailing shoes design!!! So what does the trailing shoe do? Idiots.
As an aid to anti-seizing, I've modified the system not to use cables and the actuation levers are slightly longer to allow the brakes to be used in their unadjusted state. I realise there is a greater travel required but this is acheived by a wee modification at the hitch. The adjustment screws are replaced by stainless ones. The cams I've just greased up to see how they last.
I intend to investigate linings which can be riveted to the shoes and, if they are available, I'll make stainless shoes. It should only have to be done once so not too bad a job.
I'll be interested to hear how you get on with your modifications.