I honestly don't think I've ever seen this monster used, in any of its three varieties. It's such a strange creature compared to things like orcs or dragons. Like the Cave Locust or the Robber Fly, it's got the whole "giant bug" thing going on. At 1' to 5' long, they certainly qualify. What's interesting is –as far as I can tell– their main use in a game is as an indirect threat to the PCs.
The stats: (from Cook)
Swamp/Fresh Water/Salt Water
No. App: 0 (1-4)/0 (1-3)/ 0 (2-7)
Move: 90' (30')/120' (40')/180' (60')
The special attacks consists of a toxic spray if above water, which can stun a foe (possibly causing him to sink?), and an ink cloud underwater (like an octopus). They only bite if cornered, which makes me wonder at their relatively high morale.
The fun from these guys comes from them latching onto a vessel underwater and dealing structural damage to the hull. If they chew it enough, it starts to sink! This could be a fun way to get the PCs into the water (and off the nice, safe boat) to deal with the threat. They'd have to move fairly quickly too (before the damage is done). Of course, looking at the damage they do and the Hull Point ratings, it's unlikely the termites would sink a larger vessel. Although a few leaks might make people nervous. Perhaps a swarm of salt water termites follow a ship like dolphins, feeding each night? The non-marine versions are quite scary in their own way, too. Imagine your canoe getting trashed when you're miles into a festering swamp. Personally, I'm a fan of anything that underscores the differences between an encounter on the water vs. one on land.