Some models like the Seamaster Professional have screw-down crowns.
To open the crown, simply turn the crown anti-clockwise to unscrew it (takes about 4 or 5 turns).
For more information on this, read the major section on Water Resistance.
All current OMEGA watches come from the factory with at least a 30m/100ft depth rating water resistance--sufficient to handle basic water activities like hand washing, swimming, and other light water activities.
Your mechanical watch should continue to run at up to these temperatures.