Your specification of
200-2000 Hz bespeaks a sound range of an experiment of technical nature. On this lower end of the audio spectrum, you could likely encase any speaker inside an air filled waterproof box of metal or plastic, appropriately weighted. The air filled encasement methodology shouldn't adversely affect sound transmissions in that spectrum. (Caveat: just a SWAG)
Again, due to your audio range spec and phrasing, I'm assuming it's a semi permanent installation, not for swimming about.
Most similar questions, the ones that rightfully garner the "Oh please, no" commentaries, relate to the 20-20k Hz human hearing range, which is an insurmountable obstacle of sound transmission through liquids, at least at the hobbyist level.
I have utilized USGI ammo boxes to 60fsw with success. There are many waterproof pass-thru fittings for wires available. It really wouldn't cost more than $10 for an ammo box...put 10# of weight in it (and a sprinkle of sugar) and tie it to a 100' string...give it a try from the boat. Is the sugar dry? Good to go.
The boxes got replaced every year, but seemed ok even then. The big problem I never resolved... What the hell is nibbling the wires? Switch to co-ax, let 'em eat.
Nota Bene- In the 1960's, I used a reinforced ammo case to house a Bolex 16mm movie film camera. A simple square "lens port" was cut in, siliconed a piece of tempered glass, used commercial surplus waterproof connectors, took it to 160 in Truk. Now we have GoPro and go to Chuuk. The airlines now also have weight limits.
So there's that.
