Very common misconception about bearing buddies - they have a spring on the "piston" that provides all the pressure you need, if you pump them up until the spring is completely compressed then add more grease all you do is push the seal and half your grease out the back.

Stops your stub axles rusting as much when they get coated in all the grease flying out the back of your bearing, though 

Generally we do 30minutes - 2 hours at 90kmph, then straight into the drink after tie downs are off. 

On the bigger trips wetsuits are needed to be slipped into at the ramp / beach.. 

They get put on but during winter usually at home because its easier and warmer especially at dawn

So during winter it is often 15-30 seconds before into the drink and have never had an issue. 

Like Steps said earlier, maintenance is pretty much key. Bearings are checked every 6months or so and repacked when required. 

Concerning bearing faliure most common causes are.

1  Insuficient grease at assembly (some garages think a smear on the bearing is enough.it is not. Bearing should be packed and the cavity between both bearings in the hub packed full to the stub or bearing grease is thrown into this area as soon as the trailer does 80kph)

2 Incorect bearing preload with either way too loose or way to tight being almost the norm as average Joe thinks he can set up a timken taper.

A properly packed hub and bearings set up properly should last years almost regardless of use.

Bearing buddys are good, and they dont need to be pumped so full that they puke grease but they DO need to be occasionally pumped up so that there is some spring pressure on the grease at all times.

Maxing out your bearing buddys is bad and will unnecessarily wear your seals for no net gain.

Whoa! The Specific Heat of Water and Steel are very different. Cast Iron, which most brakes are made from, and Steel, which the axles, bearings etc. are made from, have approximately 1/9th the SH of Water.

If you know the mass of your components and the temperature that they are, it is easy to calculate the energy required to bring them back to ambient temperature.

There should be very little heat build up in a properly lubricated system. Heat, in this instance, is a product of friction, which is mainly coming from brakes, but at the same time there is a large convective heat loss happening with the forced air flow over the system whilst driving.

If you have an unbraked system and your bearings, axles etc. are getting anything over 45degC, then I would suggest you have a problem. Dropping anything into water at this temperature is very unlikely to draw any water in, unless you have faulty seals.

The variables with a braked system change greatly, but on most trailer boats, I would be surprised if a properly lubricated and sealed system is getting much over 55degC.

I'll repeat; it is easy to calculate the amount of energy required to lower a component to ambient temperature. Yes, cold does not exist, but it is a simple heat transfer equation (kJ = (SH*M*Td). The big variable is time to lower the component, based on the heat transfer method.

I don't know what grease you're using, but anything that is breaking down at 55degC wants throwing in the bin. The Rocol grease mentioned a couple of times here is good to 150degC. In fact, the lubrication properties won't be that great until the grease is warmed slightly and freely flowing, to enable full lubrication. 

My boat, ~6T on trailer, with 4 x 13" disc brakes can easy get 55degC at the wheels going over some hills. Using bearing buddies, top notch grease and quality seals, I have had no problem.