Try a strap wrench. Be careful loosening and tightening so you don't damage the thru hull/seacock.
I changed to a Vetus raw water strainer attached to the bulkhead to remove it from the seacock, give me better visibility inside the strainer basket, and to have straight through access to the outside if the thru hull/seacock were to get clogged.
I changed to a Vetus raw water strainer attached to the bulkhead to remove it from the seacock, give me better visibility inside the strainer basket, and to have straight through access to the outside if the thru hull/seacock were to get clogged.