I would try to stay away from using an extension lever to the existing handle. Seacock handles are not really designed for that much torque on the handle. Especially the SS version, they're thin and if you torque too hard and are at all off center, you can bend the handle to the left or right, once that happens the handle looses considerable strength. If you have a seacocks with a bronze handle you can deform the handle or the components. Spartan actually makes their handles to break before there's too much torque to do harm.
I would definitely not use an extension lever if you have a through-hull or Groco flange/ball valve combination. Although I like the Groco flange concept
, the weakest link on either of these two options is the exposed threads, I have know idea how much stress they can take.
You might try a little heat on the seacock and a tap or two with a mallet before putting too much pressure on the seacock. If you're really desperate you can try a box wrench.
I would use barnakiel's method of using a crescent wrench (maybe a bit longer than the handle), the wrench is thicker and will allow a bit better purchase
. Again be careful, you're working with bronze or SS, both are malleable.
I would not try any of this with a Forespar seacock, the design and material cannot handle the stress, and below the waterline, it will be even easier to destroy. If you have a Forespar 93 seacock you're completely out of luck. The 93 is designed so that if the handle is torqued in the open position and breaks you will have a full flow of water
running in your boat.
I have yet to figure out how Forespar ever got its 93 through the UL 1121 tests.