Here is what I have:
Since I just took it out of the canister and stored it close to my berth SUnday, I can tell you right now the gun is not plastic. I also have a secondary set with the plastic gun (which seems a little squirelly to me). The 25M is the way to go for defense, in my opinion. I "understand" that in most countries it is not considered a firearm, but is in some (so check your rules). I have no doubt a slug in a 12 Ga shotgun shell would be more effective, but I sure would not want to be on the receiving end of that thing. It is only for close encounters anyways (ie, someone breaking on board while you are there). It would be pretty tough to miss them in 40 feet.
I am NOT for firearms (traditional) on boats outside the US. MOST counties have rules against them - with some just flat ugly (A foreign firearms license is not valid in Mexico; if you own a gun don't take it to Mexico with you. The only exception is a firearm used for hunting purposes; but you will need to apply for a special permit - contact your local Mexican consulate. Caught in possession of illegal firearms can land you in serious trouble - even if you have a license for it that was issued in your home country). As I recall, someone correct me if I am wrong, but isn't that 5 years in a Mexican Prison (whether you brought it in intentionally or not)??! Other countries are not that different.
If you are that concerned about it, go buy a dog (which can have their own entry perculiarities, incidentally). Most people are not going to want to tangle with a dog on a boat. Some countries are stict on pets... so check ahead.
For me personally, here is my attack fiend. SHe is vicious, so don't let you children see this:
If that does not work, consider hanging this picture with the warning sign: WATCH OUT!! SHE IS INSIDE!
Hope that helps (with a little humor).