1. Water resistance test. Make a water resistance test before summer and anyway before all submersions, even if the watch is a few months old.
2. The 10-atmosphere limit. Remember you should never make submersions (but it would be better not to make bath too) with watches that are not guaranteed for at least 10 atmospheres and which are not provided of screwed crown and back (about the back, other solutions are acceptable if they guarantee the 10 atmospheres).
3. The screw closing. If your watch is not equipped with screw closing avoid any "aquatic" use: even if guaranteed impermeability should be good safe, you will never be sure that crown gaskets are not damaged by a hit you could have not recognized.
4. Look at the chronographs. It is not advisable to submerge a chronograph if it is not provided of screwed crown and pushers (and of water resistant back). Avoid actuating pushers underwater even if the Brand allows it explicitly. A modest hit is enough to compromise o-ring gaskets. In all the other chronographs an unwanted action of the pushers could cause a real disaster.
5. The vibrations danger. Be careful with long motorbike trips: they can damage the strongest of the mechanical watches, especially if automatic wind.
6. Thermal shocks. A long exposition to the sun of a watch (especially with black dial) and a subsequent immersion in cold water cause a thermal shock affecting strongly its rate.
7. Look at hits. Every watch, even the strongest one, can be damaged and can stop after a hit that you could have not recognized.
8. Flooding. If your brand new water resistant watch gets flooded, do not try to dry it. You must submerge it in running water and immediately run to the next qualified repairer. This way only you can avoid major damages.
9. Guaranteed repairing. If the flooded watch is protected by guarantee, immediately ask for a water resistance test, to avoid problems. Remember that if the watch passes the test, then you are guilty: may be the crown has unscrewed because you forgot to close it properly or may be you forgot it completely open. In these cases, you could not obtain a guaranteed repairing, but you should pay for it. If, as often happens, the person who sold the watch grants you the right to obtain a free repair, do not forget to thank him, it has been very glad of him.
10. A watch can (almost) always be repaired. When you want to make a submersion, choose the most robust of your water resistant watches. Remember it is very difficult to damage a watch in such a manner that a good repairer cannot make it return as new.