Hello Gentlemen and Ladies

[TommyDee]

If you want stiff use PLA. PETG is nearly as soft was ABS. Tough yes, but not anything I consider rigid. PLA is in another class as it is a crystalline structure. I've put PLA prints outside for nearly a year in the form of a print-in-place wind spinner and it is still spinning.
Although organic... it won't last 10 year, but it will last in the water long enough to get a busy season or two in. And it print easier than anything else.There are a few slightly different levels of PLA as well, but the difference is night and day over ABS, Nylon, or PETG.
Each have their place but from what I can tell that you are asking for, PLA is where you want to start.

That is my opinion anyway I've been building some small structural housings using 3D Solutech standard PLA (real white). A little harder to stick to the build plate but it does have a little extra in the stiffness department. I run LokBuild with 3D systems CubeGlue. The glue remains with the LokBuild and I get super adhesion from this for all FDM materials I can print. Refresh with some water after a few prints and good as new. A small bottle can last me a year. And I print a lot in a year!
But that is another thing you will learn about. Hairspray is also a cult goto adhesion fix.
Then again, there is the old blue painters tape that works well if the part don't curl.One other trick it so design in internal structures... Extra walls or voids to force additional skins to be printed. Often, you will get more stiffness from this because you can add orientation of internal ribs.
I haven't had to worry too much about layer adhesion and therefore my layer orientations are less important to me than other factors. I am not certain of the highest loads you will be dealing with and what elements are required, but I will suggest that keeping water flowing with the layers will be a critical factor to minimize turbulence. You also won't have high-g shock loads, which is what really takes out parts where you depend on layer adhesion. For instance, I have better than a dozen prints that reply on printed threads. I print the screws and nuts standing up. If I only use outer skins, it is doomed. However, I put a hole through the middle, and I get 4x the strength minimum. And that is only 2 skins on the outside and 2 skins on the inside. And yes, I am using PLA for screws equivalent to 1/2-13 UNC. The stuff is tougher than a lot of people think if the print itself is fortified correctly.

I design all my parts. I have full control of the particulars of the printer. I also build in all the supports as printed pieces. A gap in Z of approximately .008-.010" will make for a good break-away support if you are not supporting large planes. You will want to make sure the print is stable on the bed so it doesn't rock while new layers are being applied. There is a little bit of drag in the extrusion process that you will have to account for as the print gets taller, regardless of material.
Relevance of the material presented is dubious only in the fact that I am too lazy to read every word in this thread. But it sounds like you have a mission and I thought I'd throw some coins in the pond.

As too all this single paragraph... that comes from a simple edit. I'm not taking it apart again.