Lots of questions...lets get started.
1. My exhaust exitted above the swim platform on my 260 but are below the platform on my 390X. Most 260's I have seen with thru-hulls have always been above the platform.
2. No swim platforms should not drag in the water as they are (or should be) over two inches above the waterline at a stand-still float, and when your underway the hull rises anyway the faster you go, and when you trim the boat properly with the tabs etc, you can get a lot of the hull out of the water for less drag so therefore the platform should not be any problem.
One instance I noticed on a 230 was a guy put a huge big block in it when it was designed for a small block and the boat was very aft-heavy. If the boat is butchered by someone who doesn't know what they are doing they may change the way the boat floats.
3. Your question regarding drawing in of water...If you have the big manifolds which are stock on Mercs I don't think this will be an issue as they are quite huge and can handle large volumes of water such as splashing from behind. I was in 12-15 foot seas one afternoon with my 260 and I crossed Lake Ontario without stalling the engines once. I had some backwash when climbing and riding down the huge waves, but no issues.
4. Radar arches are made of aluminum and if you find one good luck! They are quite rare and expensive used. I saw a stainless pipe one that a guy paid a grand for so I copied it and made it myself for 28 dollars and a can of grey paint. I welded and bent it up myself and made it fit and put my lights and antennas on it.
On 260's there are step plates on both gunwhales. If you remove them, as they are just a couple of pieces of teak, then that is the mounting spot for the Radar Arch directly behind the windshield bracket and fore of the grab bar surrounding the day bed. I drilled holed in the gunwhale and bolted my arch from under the hatch area with easy access to the nuts and washers.
5. Bimini Tops are attached with small sliding mechanisms and aluminum bars with brackets that slide and fold easily along the inside of the windshield. These parts are available at any boat shop or upholstry place as they are quite common. If your Marina has scrap boats in the back yard, go hunting. I'm sorry I don't have any closeup pics of my setup but I had the short Bimini and full camper top on my 260. It was a two-piece with a zippered centre.
This is a pic from the CC Brouchure which has an Arch.
These pics are aof my 260 and you can see the thru-hull exhaust (IT SOUNDED AWESOME!) as well as my home made 28 dollare arch!
If you look at the side shot of the rear area near the platform you can see a small pinstripe on the bottom. My waterline was about 1/2 way between the pinstripe and the base of the swim platform so it was always out of the water at all times.
My Toys...
My mini boat
This was my old Stinger 210 and I built an arch for it too!
