My trailer too has 10″ tires and I keep them at 2 Bar. Actually they are car tires rated for 140 kmh.
The pressure depends on the type of tire and the weight of the trailer and cargo. Too much pressure ant the tire wears in the middle, to little pressure and the tire wears on the sides. Only the correct tire pressure exposes the maximum amount of rubber to the road. Unfortunately it means the tires get warm, specially in summer on a dry road. This is nothing to worry about, it is normal. As long as you can keep your hand flat on the tire for a few seconds without pain they are not too hot. You can spot the correct pressure also by looking at the tires from a distance, too much pressure and they look like a ball only touching the road in the middle. Too little pressure and they look like the sides are pouring out. But it is easier to look up your tires on the manufacturers web-site. There you will find tables that give the weight to pressure ratio for your type of tires. To estimate the trailer’s weight take half the weight of your trailer and boat. (The other half of the weight is on the other wheel). A Wayfarer weighs, or is supposed to weigh, 186 kg less the mast, boom sails and rigging. Add those and some weight for the trailer itself and you’ll find that 300 to 350 kg is a well educated guess. Putting her on the nearest lorry scale gives you an exact answer of course.
On motorways (British or German) I would prefer tires that are rated well above the highest speed I ever expect to drive (lawful or otherwise). New trailers in the UK seem to be fitted standard with a sort of wheelbarrow tires, rated for max. 20 mph or so it seems. These tires are OK to drive down the road from the garage to the local sailing club I suppose but you wouldn’t be the first to blow both tires on your first trip to Denmark with your brand new trailer……..
Anyway, I don’t worry too much about tires. A far bigger problem IMHO are the hubs or rather the bearings of those hubs. Do keep them well greased. The seals of the bearings wear and need to be replaced every so many miles or years. Bad seals allow grease to ooze out and just as bad, (sea) water to get in. I have never allowed the axle of my trailer to submerge just to be safe rather then sorry. Now that I own a combi-trailer I never need to any more. A trolley has no bearings other then the nylon the wheels are made of and can be submerged without having to worry if the trailer will make it back home all the way from Copenhagen.