I bought the drain pan (roughly 18"x25"x3") online from WebstaurantStore.com for about $40. The drain itself came from Lowes as a drain plug and I had to drill out the center hole ($3-4). I even used a heat gun and carefully heated up the plastic and used the drain to pull down and form a slight depression for better drainage. Bought the 1/2 NPT to 5/8" barb at Ace for a few bucks.
I unbolted the AC unit but left everything hooked up. I slid it to the side so I could measure for the hole locations for the plastic pan. This must be relatively accurate or you won't get the bolts back in. I then drilled drainage holes in the bottom and sides of the existing metal pan so that the condensation would drain into the plastic drain pan. I picked up the unit and slid the plastic pan underneath. I used some rubber washers under the bolts to seal the bolt holes. As you can see in the pics, the metal pan will continue to rust out but instead of the evaporator sitting in 3 " of water and rusting, that water will drain out, hopefully extending the life of the entire unit some 3-5 years. Also, I don't have water draining into my carpet.
If I bought a new AC unit, I would rebuild it something like this before I installed it or find a unit that was properly designed to avoid all of this. Instead of metal, a slightly larger hard plastic pan with a bottom drain would be the correct way to build the AC unit. If someone is buying a newer boat, they may consider this modification early on before the existing metal pan rusts out like mine is rusting.