No worries about a 316 nipple welded to mild steel with 309. The welds do not suck and are easily done if you know how. I'm not sure where you guys are getting your info but you obviously have been led down the wrong path. If you have had weld go bad in a 'lifter'... and I think you are referring to a riser,,, then it was probably done with the wrong material or rod. if you build it out of 304 then you will have a problem. if you use 304 0r 308 rod then again you will have a problem. If you use 316 rod or 304 ,308 and weld stainless steel to mild steel and subject it to salt water
you will have a problem. If you are using 304 for your base material you will have a problem...especially when you run hot salt water
through it.These are all dissimilar metal troubles that can be worked out by using the right material for the right job.
Same thing for black water tanks
....304 stainless and urine don't agree with each other.
Unless the "professional welder" is thoroughly knowledgeable with marine
welding and materials he will use the wrong thing and you will get undesirable results. They will look "pretty" at first but later after use they will experience electron transfer --(simplistically one alloy is robbing material from the other)-- and you will see corrosion, cracking,pinholes ,etc.
You will see this when welding ss stanchions to decks,,hardware,, hauser rings etc.
Basically just because its stainless doesnt mean it is fit for the marine environment
....it should be 316 alloy and above.
I wouldn't be a fan of using plastic through hulls on my steel boat. You bought the boat for the strength of the hull,,, why compromise its integrity with a marelon through hull that can shear off in a crunch. Just seems silly