SEAC SUB CASSIA

I don't have the gun but it will be very similar in performance to any other traditional pneumatic out there. So, anyone else who is hunting with an airgun in the same conditions as you can be a guideline for the length. Or tells us what type of hunting you do and perhaps we can help.

Other than that, I think the Cassia may be an 11mm barrel gun? If so, that's an advantage if you don't want to put a dry muzzle on it. An 11mm gun will have a more powerful shot with a wet barrel than a 13mm gun simply because there's less water in the barrel to push out during the shot. If you put a dry muzzle on it later on, then it doesn't matter much if it is a 11mm or 13mm gun. Just keep in mind that the easy-to-get Salvimar Vuoto muzzle would not work on a Seac gun as the thread on the barrel is different.
Actually, on that same note, if my research is correct a Seac 11mm gun is one of the hardest ones to get a cheaper dry muzzle upgrade for.

I have a Hunter which I think is the 13mm version of the same gun. It's alright. The plastic parts are lighter than e.g. Salvimar's (I think) so it shouldn't really be too heavy in the water. I don't particularly like the shape of the handle but that may just be me and you can modify that down the road if you feel like it.

I think my Hunter had a 2mm trigger pin which I later swapped out for a softer 1.5mm one but 2mm is not too hard. Guns used to have 3mm pins so 2mm is already a lot softer. A Predathor from Salvi has 2mm, too whereas Stens has 1.5mm and the Cressi SLs are still stick in 3mm-land.

As said, this gun will shoot exactly like most other wet barrel guns so I would only get it if you prefer the looks or it's better priced than the others.

The Caccia 90 should be OK for your type of hunting, I bought a Shotgun 90 which is the same gun with much less air tank. It has since been discontinued as the gun is nose heavy without the full length tank when cocked to shoot. Only bought it because they were selling out cheap and was curious about why they made it.

I think I saw you mentioned something on another thread about wanting to shoot into holes, too?
Keep your 40 for that, get the 90/95 as a good allrounder which, on the low power setting, I would still be OK with taking a shot near rocks with.
The 115 is getting into large gun territory for air guns and starts getting a little large to swing easily on the reef and you will be worried if there's a rock right behind the fish - not so much with a 90.
If you were shooting amberjacks or very shy reef fish far away, then maybe yes to the 115, but a 90 airgun really is a nice allrounder.
Coincidentally, a 90 Seac Hunter was my very first gun and I still think it's a great reef gun.

But like I said, a Cassia would have to be a fair bit cheaper than the competition for me to fully recommend it.

Here's an old video of a sailing trip I did, where I managed to get a little bit of spearing done. My gun is the Seac 90, but it has been upgraded with a vacuum/dry muzzle. But you will get a sense of the size of the gun and a bit of the distance of the shots: