It makes sense, but I wouldn't rely on it. Current pseudorandom number generation kinda works the same way; you just seed entropy from random peripherals until you have enough randomness to pretend that you're random. It's not a perfect solution, but the "kitchen sink" nature of PRNG seeds makes it hard to crack.
The problem with seeding natural patterns like water anistropy is the fact that it's still a pattern. Depending on the time of day, large bodies of water will vary their chop and present more or less sparkles depending on the position of the sun/moon. If an adversary wanted to bruteforce your sea-sparkle cipher, they could reference recent moon phases and weather to narrow down the potential entropy that seeded the key.
The problem with seeding natural patterns like water anistropy is the fact that it's still a pattern. Depending on the time of day, large bodies of water will vary their chop and present more or less sparkles depending on the position of the sun/moon. If an adversary wanted to bruteforce your sea-sparkle cipher, they could reference recent moon phases and weather to narrow down the potential entropy that seeded the key.
It's a cool idea worth building, but still a toy.