USS Richard B Russell SSN687/Sturgeon Class has official operating depth of greater than 400ft.

1 fathom = 6ft, so 66.66 fathom operating depth.

It's not possible for Ketron island to house this sub class, the water is not deep enough to navigate (submerged) through Puget Sound all the way to the island. North of the island, up in the traffic lane, water is deep enough, but not farther south towards the island.