Re: 6wt Saltwater Reel for Sage Xi3
I fish a kayak in the salt frequently, and nothing I've ever done gets a reel as dirty. (muddy boots in and out of the boat WILL get the boats cockpit dirty)
Picking a reel with a sealed drag is a GREAT idea, and I knowing how dirty the thing's going to get, I wouldn't spend tons on it.
I use a Ross Evo LT on my 7wt most. Its the size 4 and supposed to be for 7-9wt lines/rods, but its slightly too light for my 7. I wouldn't hesitate to pair it with a 6wt.
Another wonderfully light option is Nautilus' FWX. You could go with the 7/8wt size, have tons of backing capacity and very fast retrieve (due to arbor size) and still have it balance your 6.
Anything from Lamson would also work well, due to the completely sealed drag. They get lighter per their capacity/retrieve-rate as you go up in price, so choose accordingly.