I honestly can't tell from that blurry photo whether or not the valve is opened or closed.
The sea valve as you call it is the raw water intake line, & the valve handle should be parallel or inline with the direction the hose runs. Perpendicular to the hose is off. When in doubt remember "righty tighty, lefty loosey." If you ran the engine for more then a couple of minutes with the raw water cooling intake valve off the engine will quickly begin to overheat. Always check your stern transom water discharge before you leave a slip or begin motoring to verify that water is discharging out through the motor, to the water muffler & out through the stern. If you don't see water after a minute or less then shut off the engine immediately & find out where there is an obstruction to water flow. Most 1980's era sailboats have a very simple bronze strainer basket welded to the exterior side of the hull at the water intake thru hull. It can easily build up barnacles & obstructions if she's been sitting for a while. If the engine ran with no cooling water running through it, the rubber impeller at the water pump will burn up within a few minutes of overheating & will need replaced. If the engine was smoking then the diesel was pretty close to enduring major damage. If she still starts up next time you try her, then your head gasket & seals survived the ordeal, then you'll likely be o.k. I'm assuming that you always check your oil level, coolant level & verify the water intake valve is open before running the engine. Your engine looks like an M 25 so your fresh water cooling heat exhanger could also be clogged with rust scale /salt corrosion.
Good luck on your fix.
