Seal spawn 1 on big map. There is a metal gate. Its identical to the gate at seal spawn 2 but you cant open it.

Be at seal spawn 1
nothing needed
run towards it
lay down while running towards it
when your laying down do a 180 turn
back up into it perpendicular
keep doing it
after a few seconds you will back thru it
then follow the hallway to outside the map
and go swim !