I'd consider a couple of things: a high-quality, wired-in surge suppressor (well, if the tub was actually plugged in, I guess you might be able to use a good plugged in one), and if really paranoid, you could add a properly engineered, grounded pole near the tub to act as a lightning rod. This won't protect from a direct strike, but should resolve near misses and minimize near misses. They provide a cone of protection underneath them - I forget the angle, but the higher it is, the larger the diameter of the protection as that cone intersects the ground. this is what we did to protect things like radars and missile launchers when lightning was a problem (missile sites often ended up on the top of mountains!). Get a surge suppressor with the highest joule rating you can afford, and fast response (the better ones are around 1-nanosecond). To work, generally, it needs to be still connected to the power ground in order to have a path to discharge. The better ones come with a decent insurance policy against damage...just read the fine print to ensure you qualify, and keep whatever materials you need to justify the claim (receipts, etc.).