T-handle type seacock.

I have a t-type seacock for my genset fresh water intake. It looks somewhat like this without the large handle in the back.* It is difficult to get to and to turn. Is there a tool I can use to help close this valve so I can service the genset?

I had this style in a previous boat, you had to unscrew the rear T handle before turning the on/off handle. Then tighten up the T handle again. The on/off handle would not turn until you unscrewed the T handle.

Loosen the T then turn the handle and retighten the T

I will give it a shot do not see the handle just the T-handle. Its possible it is hidden.

The inside is probably a tapered cone, sometimes rubber but often bronze. It needs to be loose to turn

It is a Groco SV series seacock. There is a large rubber cylinder in it with a hole through it. The large handle turns the rubber plug to align the hole with the thru-hull to allow water to pass through. When you tighten the T handle it presses on the rubber causing it to expand and lock in place.

Turning the T handle won’t turn the rubber. To open or close the seacock you need to loosen the T handle and turn the rubber plug with the big handle.

There are no parts available for these. They were discontinued at least twenty years ago. You should be thinking about replacing it next time you haul.

Here is a link to the owners manual for that seacock.
https://www.groco.net/media/file/file/s/v/sv-series.pdf

I used a silicone grease to lube the rubber inside the seacock to help it turn easier.