There are three seals for the bow thruster on my 1987 Maramu.  2 - foam seals that seat between the thruster foot and the bottom of the cavity that the thruster retracts into.  Then there are 2 large O rings set into grooves that are only visible once the bow thruster is removed.  On top of the inside of the hull is 1 foam seal that is compressed when the BT is in the down position.  To replace any of the seals, you will need to remove the foot of the bow thruster.  

   It would help if you would post your name, vessel name, type and hull number.

Best,

James

SV,  Sueno,  Maramu #220