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.