A good trailer should be able to handle immersion as long as you take reasonable care of it. All the failures I've seen stem from neglect, not immersion. Before launching, you should:

1) Disconnect the trailer lights. Boat trailers are made with submersible lights, but they are not meant to be illuminated underwater. If a hot bulb contacts cold water, it will crack and burn out. You also don't want the water to short out your vehicle lighting and blow a fuse.

2) Always let your trailer sit for at least 15 minutes after towing before launching. It is critically important to let your hubs cool. As you drive down the road, they get warm, and if you dunk them in cold water, the warm air and molten grease inside the hub will contract. That in turn will draw water through the seal into the hub. Once inside, the water will never come out (short of repacking). The bearings will then corrode and fail prematurely.

If you launch in salt water, you need to thoroughly rinse (or better dunk) your trailer in fresh water at the first opportunity. Dried on salt attracts moisture from the air and greatly accelerates corrosion. Your trailer should also have sufficient drainage so as not to trap water.

And, of course, don't neglect basic trailer maintenance, such as regular inspection, checking lights and tire pressure, and annually repacking the bearings.

Regards,
Eric