The first SEAL use of S&W revolvers was at the very beginning, when they used standard US Navy S&W Model 15 revolvers.
They'd asked for Model 19 .357's, but a supply officer "saved the Navy money" and gave them the Model 15 in .38 Special.
There was some use of the S&W Model 36 and Model 60 Chief's Special as a concealed carry gun.
When SEAL 6, the anti-terror unit was formed, Richard Marchinko, the first commander, made sure they got the S&W Model 66 Combat Magnum because of it's ability to handle immersion in salt water and still function.
In his book "Rogue Warrior" Marchinko goes into some detail about the SEAL's problems getting the weapons they needed, and the efforts to get the Model 66.
He also shows pictures of SEAL's using the 66 in training, including during a practice take-down of an off-shore oil rig.
Later the S&W Model 686 was bought, and is likely still used for special purposes.