MT,
I don’t think a spray skirt would be necessary or help much. The SeaWind looks like it’d be pretty dry and if you were shipping some water you probably shouldn’t be out there. I don’t really know. Maybe if it was rigged up to another SeaWind, catamaran style, you could use one??

I do know that kayaking you seem to be paddling in the water, not over it, so you dress accordingly.

Lake Superior,
It can be rough, but early summer seems to be a better time to go, mid-June to late July. The cold surface water keeps the air near the surface cool and acts like a thermal blanket deflecting the warm summer winds from reaching the water surface. It seems to break down near August and you’ll typically have more layover days later in the summer.

Isle Royale,
There’s a long shoreline from the south end clockwise around the west side where there doesn’t seem to be any safe landings. It’s a long run to make without any escape routes. I’d look it over very closely and research some first hand info.