There are legends about a guy from Brazil who ejaculated 42 times during the night and died of dehydration. Of course, you still have to try to die in this way. Personally, I doubt it was dehydration. Even if he ejaculated with a maximum of 10 ml each, the total volume of fluid loss was 10-15% in a few hours, this is actually not enough to cause death.
There are no risks of masturbation as such. Illiterate and stupid people spread stories that brains leak out with semen, girls become frigid, hair grows on their palms, etc. The harm can be only indirect: infection of the genitals (with a variety of things up to the hepatitis B virus), pathological attraction to masturbation and substitution of sex for it, genital trauma due to excessive intensity of the process.
Tips can be given as follows : 1) wash your hands and the straps themselves before and after and 2) do not be ashamed of your addiction. After all, almost everyone on earth is doing this.
On the contrary, it is useful, just change your hands. The muscles of the hands are strengthened, the skin on the palms becomes denser, and determination is strengthened. I have met such, you can see them from afar, they are calling for nerds.