The bog standard aluminium power resistors should take water ok. Aluminium case, filled with resin at the ends and tinned terminals.
Don't bet on that. It only takes a tiny seepage through the seal as they cool down and next time you switch on it will puke its guts as the droplet of water that got inside turns to steam.
If you *MUST* use aluminium case resistors, the sane thing to do would be to mount all the resistors to a heavy slab of aluminum as a heat spreader and interface that to the bottom or side of a rectangular stainless tank (think catering equipment suppliers) with thermal grease, stainless bolts fitted from inside the tank with gasket paper washers coated with hermatite red (or potable water jointing compound if you want to make tea!) under their heads, nuts on the outside directly to the tank, then oversize holes in the aluminum slab, big, lightly greased penny washers to bear on the aluminum, then a stack of Belleville washers and a locknut to provide contact pressure without rigidly fixing the aluminium so its free to slide slightly against the stainless to allow for its differential expansion and contraction.
However if you can import 120V water heater elements within your project timescale, you can almost certainly do it more easily with less machine shop work. They just need a deburred hole in the tank bottom or flat side + as they are usually intended to be mounted in a NPT threaded boss, you'll need some stainless nuts with the right thread to go on the inside.
Another idea - if the frequency range is reasonable, boost the voltage with transformers then use a three phase Variac for power control and run a cable to bathroom heaters mounted on the exterior of the building, with a small roof over them to shield them from direct weather.