You can't supplement it with your urine, because your urine will be containing the salts you're trying to get rid of.

If you had an ample supply of urine from someone who was extremely well hydrated, maybe.

But yeah no you shouldn't be drinking seawater at all, it's just too salty. You're expending more water of get rid of the salt. Coffee or tea would be fine despite slight diuretic effects, but ocean water is just too salty.