As sexy as sex in water may appear at first, keep in mind that it's one of the more risky kinds. Here are some of the issues:

1) water will wash away natural lubricant, use silicone type to make up for it - you already know this
2) as Mr.Pitt mentioned, silicone lubes can't be used with silicone toys, because it will pretty much melt them - that means the safe materials you're left with are stainless steel, glass, treated wood, and certain types of ceramics. Anything that isn't silicone and vibrates is likely a hybrid silicone toy or outright a toxic type like jelly or tpr. I strongly suggest you avoid them; go do some googling on the topic to find out why.
3) sex in any kind of water poses a higher risk for infections
4) sex in water makes water get inside and often creates an uncomfortable amount of pressure against the uterus with all the movement. While that may sound appealing, most women genuinely dislike it. If it's rough enough, this could potentially even cause permanent damage to the woman without either party even realizing it. There have been cases in which women became unable to become pregnant later down the road because of this.
5) I could see a smaller vibrator being inserted and not moved around, though - just to please the g-spot a bit. You could technically do that with a silicone toy, with water based lube to get it in, and just leave it be - no movement. That or you could have her wear one of those wearable vibrators while you focus on the clitoris.
6) remember, sex in porn shouldn't be an everyday couple's standard go-to instruction manual. It's usually created to satisfy the male gaze, not the female body, and both people in the film are paid actors.

As such, chances are, your best bet will probably be just sticking to a clitoral toy of sorts, honestly. Either a bullet vibrator or a pressure-wave toy. Or maybe even a waterproof wand. The Maximus Wand is my most favorite in that regard. If she hasn't had the chance to try a pressure-wave toy, I strongly suggest those; they're absolutely amazing. This way you can avoid all the problems with lube and the aforementioned pressure issues n' stuff. If you must use a penetrative toy, try to find something smaller than you would usually use so that you don't accidentally injure her.

Please don't take me wrong or anything, I'm just trying to keep you both safe while you have fun, so don't take it the wrong way. In the mean time, here are some of my most recommendations in terms of (waterproof) clitoral & wearable toys, in no particular order:

Clitoral-external (pressure-wave/bullet vibrator type):
Satisfyer Pro 2
Satisfyer Pro 2+ (this one vibrates at the clitoral portion as well as the pressure-wave stimulation)
Womanizer Premium
LELO Sona
LELO Sona 2
LELO Sona 2 Cruise
We-Vibe Melt
LELO Sila (this one may pinch at the vulva if there's too much movement, plus it's hard to use with penetration, but that MAY make it good for this specific situation; see my review on it here for more details.)
We-Vibe Tango Bullet


Wearable/G-Spot:
Esca 2
Elektra (this is also an e-stim toy, so be careful!)
LELO Hula Beads

BONUS: This is a generally immovable toy that has a clitoral pressure-wave portion AND a rumbly G-spot portion - it's absolutely divine at ripping orgasms out of a good portion of the population, including myself: Womanizer DUO. Don't need to move it at all to get orgasms.

Technically you could also consider other rabbit vibrators, as those would be something you insert and don't move, but they're generally a hit-or-miss, because everyone's anatomy's different. Good luck! less