I've towed 6000lbs of boat/trailer with surge brakes without issue.

As far as dipping the trailer in water - the electric controller for the brakes is usually on the tongue so I don't think you'd have any reason to disconnect it.

Pic for reference:

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12 X5 M - 06 X5 4.8iS - 03 X5 4.4i - 03 M5 - 02 X5 4.6is - 99 M3 Dinan S3 - 98 M3 - 92 850i-6 - 92 850i - 91 325ix - 89 M3 2.5 - and a few parts cars