Re: trim sender
FYI: My experience is with Volvo P DP drives yet am assuming they use a common sender etc.
Other than the chance of loosing your tools there shouldn't be an issue adjusting it under water etc. I've done mine in the driveway - it's just two small bolts (8mm on my DP drive) that you slightly loosen and then rotate the sender clockwise or counter etc.. With ignition key in the off position, Make your adjustments in very small increments - tighten then run drive up and down fully a few times to check both upper and lower readings. Mine was very finicky. On my unit, adjusting with the key on did not allow me to see movement in the gage - so don't expect that etc..
Good luck