I don't fully under the syntax of the Spawn Tag and have only found vague information on the wiki. Does anyone have a link they are able to provide that has a detail explanation of the syntax?

I have an issue with Watercreatures spawning on land and in solid blocks when i use the default configuration and when I use the example on the wiki. I would like to try a configuration that will spawn a watercreature ONLY if a water liquid block is detected above AND below the block to spawn in. This should eliminate all my issues if the spawn detection is functioning properly.

Any thoughts on how to eliminate underground pool Watercreature spawning?

The Watercreature spawing issue is a constant source of server crashes as I repeatedly have to manually edit region files to clear chunks of improperly spawned fish.

The creature check should already checks for above & below.

You could try to apply that to the entity level with tags.i.e. "Spawn Tag": in the config file.

https://github.com/ProjectZulu/JustAnotherSpawner/wiki/Tags-%28New-System%2C-Post-1.7.10%29#time-functions

If you're not already using them I'd combine ent.modspawn (to get the default conditions) with search.liquid to search the area for water.

Can the liquid and sky perimeters be used in conjunction or does the presence of liquid eliminate the ability to detect sky? The objective is to eliminate underground spawning of Watercreatures.

You'd need to test to be sure. Use /jas canspawnhere https://github.com/ProjectZulu/JustAnotherSpawner/wiki/In-Game-Commands#analytics-commands

The current implementation uses the heightmap to determine if its 'above ground'. I do not know if the MC heightmap includes liquid blocks. My guess would be yes.