Do "water bomber" aircraft pilots have more relaxed rules?
All aircrews have to comply with laws and regulations, unless they have a waiver that makes them exempt for certain regulations. Aerial Fire Fighting need to fly lower than the minimum safety altitude in order to pick up water, so they will have a written permanent waiver to do so. 

This is similar to ATO's conducting training for student pilots. They can be allowed to fly below the minimum safety altitude (in Germany: 500ft) for the purpose of training engine failures and attempted emergency landings in fields.



In all honesty, it does not look like recklessness to me, it looks like the pilot miscalculated his approach and came in too low and with not enough speed, as he bounces off the water surfaces instead of doing a proper flare.