I don't know about stressing out the heart or a healthy heart but soaking in water that is above body temp is going to act as a vasodialator for every vessel in your body.

Couple that with even mild dehydration from alcohol and sweating, I'd imagine his blood pressure did swing and his Vax heart couldn't handle it.

Vasodilation via hotub or sauna for a healthy person would be no different than working in the yard on a summer day though.

Good to get oxygen and nutrients to the damaged muscle tissue without exertion in a healthy person aiding in cooldown/recovery which is why people like it in the gym.

Same for ice baths. Playing with vasodilation/vasoconstriction puts a big coefficient multiplier on recovery time.

Unless you do drugs, drink, and get Vax 5 times and then randomly submerge your entire body in hot water - then apparently the dilation just kills your weak heart.