Hi - just a few comments from engineering perspective;
In this case of simple seat bench box gravity drain, the choice of fitting material or any fitting at all is almost 100% aesthetic and personal preference. In many sail boats these type drains are just clean thru holes with no fittings at all. My guess is that Parker selected white plastic because it looks fine, low cost, unobtrusive, lightweight and does not really discolor if the right grade. Downside to plastic is that it is not as strong as metal and gets brittle from prolonged exposure to UV and thermal cycling. Hence yours are cracked. Again given relative weakness of plastic it could also be impact at a dock or something. I could riff on about sst vs bronze etc. but more personal.
For adhesive sealant etc. and main question posed; make distinction between the purpose: structure vs seal.
In case of plastic, you really only have mild structural requirement necessary to hold the fitting in place. No seal is needed.
In the case of sst or bronze, structural requirement goes up a little bit not a lot and now seal becomes needed to avoid long term exposure to salt water which is corrosive.
So gasket is out in this case unless there is some mechanical fastener or u r going to glue the gasket in on both sides which is counterproductive.
Adhesive is great solution if u choose bronze or sst. It will provide the structure and water seal. The main st important thing by far is adhesion between the part and the hull, rather than the "strength of the adhesive it self".
The loads are probably a few pounds max., essentially the inertial weight of a fitting being jerked around at sea. Sealing and corrosion prevention is much more important. In that case you want to choose something that will really bond and seal to both materials, is clear or not going to look sloppy and has some give to it (as opposed to being very rigid and brittle). Adhesion is best when the adhesive "whets" which is like the difference between a drop of water on a well waxed car, it beads up and rolls off and there essentially is no whetting at all. Conversely think of ink on paper very high wetting.
One of the main features of 5200 is that they chemically cure with moisture, like cement or underwater pool epoxy. It's chemistry designed to get good whetting on many common marine materials and surfaces, like gel coat fiberglass hull and metals. It's also made by 3M who definitely knows what they are doing and have reliable quality assurance and test information. See data sheet here.
Technical Data Sheet | 3M
It cures flexible and comes in white. You can see in the tables the strength: some 252-352 psi for bronze that means that if the surface area u r gluing is small say just the edge perimeter of a fitting say 1/10 in 2. Then you could hold 25 pounds. Probably 10-100 or more times your requirement. Therefore what becomes key is surface prep and material selection. Notice two things: Do not use alcohol it messes with the moisture curing. DO use 200 to 400 grit to clean and roughen the surfaces. 5200 does tend to be messy and you need to make sure you get a good volume bead and probably ideally suited for things like they hull transducer, could be considered overkill for a plastic port install, but who cares?
So 5200 is like the Cadillac solution. There are probably 50 other solutions.
Clear silicone is probably better in most regards except maybe strength. Silicone "whets" better than almost any other adhesive. Ever notice how sticky it is?
It also is super rubbery and therefore seals really well, which in this case is the main goal.
So these issues sort of come down much more to personal choices and attitudes vs technical engineering applications.
If money is no issue and or u have a tube of 5200 and u don't care to spending time considering things like this, then use it and forget about it.
As for me, I do stuff like this for a living, and every case is a learning experience for the next etc.
I'd probably use whatever I thought will work and I have lying around.
Generally selection of fitting depends for me on an understanding of the OEM and if u think they chose the right solution at the factory. Cost almost always is more important to a manufacturer and so in many cases when we replace stuff we do a proper upgrade. But that is not always the case. I drive older Mercedes, and 99% of the time, the best approach is to replace with whatever the German engineers did in the first place. Same holds in vessels. My friends Viking has almost 100% polished sst fittings. In that case I'd just replace what was there.
If you're into restoring exact factory then get white plastic and silicone it in.
If u want viking style then get a $72 sst fitting hand made in Italy and bolt it or screw it in with a custom cut 3/32 inch thick viton gasket.
If u want reliability, just clean up the hole and have no fitting ( I've had about 3 sunfish sailboats, which are like Volkswagen beetle designed for ultra simplicity and longevity, why use a port part that really adds no functional benefit at all?
Bronze is "cool" but it's primary benefit over sst is cost but not that much and it obviously does not rust, but it sheds green. Bronze was used before SST became available. That can be cool on the side of a brick building or on an old wood 4master like the USS Constitution, mostly because it matches all the other fittings.
Parker mostly uses SST, so bronze does not match 2300DV (I don't own one but my friend does, so do not strike me down if I am wrong) but can be necessary like with some thru hull transducer that only comes in that material.
If u take away anything it should be that good surface prep is number one when doing any adhering job.
LMK, but that's about my 2 cents on a boring rainy sunday
