non toxic acrylic grout sealer?

i made a home project that will be submerged in water. to strengthen it i covered it in grout. now there is a product called Shields-all .. i dont know if its water safe . i tried using aquarium silicone + water mix. but it didnt work out too well.. the silicone was too solid for me to spread

http://www.lizard-landscapes.com/How-to-build-a-Castle.html

click that link to see what i have built. if you fast forward to where he uses the shields all you’ll see that it has to be water-like….

now my question is .. what can i use besides shields all (if i can even use it) to seal my castle. its going in water so it needs to be non toxic and acrylic !

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