Seems like your 300E belt slips when you drive through the puddle of water. When the belt tensioner gets loose or slippery, the air-con compressor would cut off, preventing engine load. Anyway, there is only one long belt(air-coin, alternator, fan etc...). It occurs quite often to Merc 300E's. The air-con is sensitive to engine timing as well. One other problem that leads to A/C compressor cutting off is vaccum leak.
Solution : check your belt tensioner, maybe time to change!!!
Alternatively, if you do not want any of the above causing A/C compressor from cutting off, you gotta get somebody to modify your A/C relay so that it does not have any feedback sensing.