I'm obsessive, so take this with a grain...
1.) Make sure you contact Grady for the bunk placement & positioning on a Marlin hull. Whomever is doing your lift may be knowledgeable, but make sure.
2.) Your lift capacity should be at least 15k. (Mine is 16K.) Your Marlin will come in around 12K, mine is 12.5K, loaded with fuel & gear. You want a safety margin regarding weight cap and it's not that much more expensive.
3.) I prefer 20 ft. bunks, which I have for my lift. Others go with 10 or 15 ft. To me, the more hull that is supported, the better. The bunks need to contact the hull as far astern as possible so there is little "overhang." (Grady recommends 3-4 inches from the transom.) Also, make certain when the bunks are being set and the guide poles are positioned, that the bunks are not resting on the trim tabs. (Mine are close.)
4.) RE: Washing/Rinsing:
I spray my whole boat & hull down every time. While I'm leaning under the hull, I spray the lift off as well. Since you have bottom paint, it may not be necessary to spray the hull, but spray any part of the boat that got hit with saltwater underway. Regarding spraying the lift; you have the hose in your hand; it only takes a few minutes and it will extend the life of the lift. (Re-read my opening statement....)
5.) I don't recommend leaving the lift submerged while your on your boat trips. It will just degenerate faster over time and a remote by your helm will lower it for you while your circling around, waiting to drive it on.
6.) I have found it extremely helpful to lower the lift to the point where your Marlin free-floats and at that point, mark one of the stern guide poles with electrical tape. This way, you don't lower the lift any further than you have to when you are ready to drive the boat on.
7.) It is a $ 700.00 option, maybe more nowadays, but the "Hi Speed" electric motors are a big time-saver when raising & lowering the boat. The slow speed models give you time to go have lunch while the boat is raising up or down.
8.) Lastly, I stand by flushing your engines after every trip. (Re-read my opening statement.) Even in brackish water, flushing outboards extends their life.
Lifts are marvelous things. I would never leave a boat slipped in the water full-time.