the truncheon is the one to get. no calibration, waterproof, and it doubles as a stirring stick. its $100USD or so, though someone on here got one for $40USD off ebay.
I've seen less expensive ones, but they require calibration fluids. which are a hassle and generally expensive. That's what made me go with the truncheon.

Absolutely right. Unless you just want a headache, get a truncheon.