Over the years before and after owning an Aerotec I've read lots of posts on splash over the transom. I think its agreed the root cause is the way the water flow from each side of the V-floor meets and bursts to the surface bouncing off the outboard leg and over the transom into the boat.
The degree of splash seems to vary from mildly annoying to a real flood.
Here's one past thread... bombard aerotec transom height
And on post #7 of this thread Chipko shared an image of a splash shield he'd seen... Old used Aerotec impressions.
And Max tried a couple of mods in this thread... Bombard Aerotec ready for the maiden voyage (b@stard son of a Y Class)
Last year by adding transom raiser blocks I eventually managed to get the splash to an acceptable degree while minimising ventilation on turns.... a best compromise. It did seem to completely stop this issue though I'd need to block the transom up to a degree that would need inner/outer outboard pads raising... and to accept some ventilation in tighter turns.
Well this year mine has stopped splashing for a very unexpected reason... an outboard change. When we first used my new Suzuki DF20 EFI over the weekend transom splash was the last thing on my mind but as soon as we'd done enough running in hours to get on the plane I suddenly realised... no splash.
(To be totally factual I did have on one of the transom raisers from last year which I thought was a 20mm but just measured to find it's a 10mm... not enough height to help with the old Mercury)
In the combined image below you can see the much larger anti-ventilation plate on the Suzuki (left) than my old Mariner 15 (right)... and that it starts at the leading edge of the leg compared to the Mercury's starting close to the trailing edge. Perhaps more important is that the upper water deflector plate is also much larger (more so than is clear from the image). In fact the upper plate is getting well on the way to the size of the wooden one Max made that fell off.
I've added a second image taken from the net that shows this better.