You can build drive and value for the toy, to some extent, but if you have a dog who's not tremendously interested in the toy to begin with, it'll never be as strong as if you were working with a dog who had a genetic predisposition to love toys.
I won't say that you'll never be able to get Zeeva to love toys enough to jump into a pool for them, because I don't know you and I've never met your dog and I don't know where the ceiling might be for you guys.
But I will say that it might be pretty hard. Neither of my dogs has very high toy drive and if I threw a toy into water expecting them to go for it, they'd just give me a look like "that was dumb, what did you do that for?"
If you're having success with luring her to swim around a little, it might be more fruitful to just keep building on that small success rather than trying to switch gears completely. Adding duration onto a behavior that you can already get is probably going to prove easier than convincing her to work for a motivator that she doesn't value at all in those circumstances.
ARCHMX TDCH Pongu the Insane, CD-C, RE, RL1X6, RL2X5, RL3X2 (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, ITD (Akita mix, b. Jan 2011)