Q. When while swimming in a lake one urinates, can he say Asher Yatzar? Does he have to wait? How long? (his head is covered by a diving mask).

Horav Hirschman and Horav Pam Shlit”a mentioned that when swimming in deep water one should just wait a bit, wash hands and recite Asher Yotzar when his head is covered.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a