The tuna mixture keeps 2 days refrigerated; assemble and griddle fresh. Don't store assembled or cooked melts — the bread is unrecoverable.
Oil-packed has more flavour and a silkier texture; water-packed is leaner and takes mayo better. Either works — drain both thoroughly.
Broiled open-faced melts the cheese fastest and keeps the bread crisp. Closed in a pan is more like a grilled cheese and takes longer. Both are in the recipe for a reason.
Drain the tuna hard, don't over-mayo it, and toast the bread first. Those three fix almost every soggy tuna melt.