Jacksonville beaches are nice north and south of Jax, but not great where it's all built up. The brown water seems to be a turn-off and it's chilly compared to the rest of FL. But it's warmer than the Pacific, that's for damn sure.
The sea turtle nests are cool, if you're into that.
I have fond memories of living in Jax from '89-'94. But every time I go back, more and more of my memories are paved over or replaced.