yes, well, better safe than sorry, i wouldnt leave them in water unattended, you should watch over it. most bathtubs have an angle, so you can fill it to a level that the BP can swim on one side, and relax in shallow water on the other side. i never said it should be done unsupervised.
if you check water with a temp gun, the readings might be off, but the temp gun measures the amount of incoming infrared radiation. and water will give off infrared radiation depending on temperature just like any solid. so when two buckets of water give the same reading with a temp gun, they will have the same temperature. im not sure, maybe the temp gun will give you accurate water temperature, but if it doesnt and there is a difference between measured and actual temperature, that difference will be the same for any water surface you measure. i never tried it, i have to give it a try (now where do i have an oldschool regular thermometer for reference?), but i guess it will do just fine when measuring water temperature.
character also plays a role, some love it some hate it. clearly there is no reason to give these that hate it an opportunity to bathe.
which reminds me of something..... CATS!!!
http://www.youtube.com/watch?v=ctJJrBw7e-c i couldnt resist

(a more serious feline side-note: some cat breeds, especially sphinx or hairless varieties, really benefit from regular baths, and somehow cats of these breeds seem to generally love it, like the sphinx at 0:30. so i think its genetic.)