The gel that is used during ultrasonic imaging is also a nice sploshing substance.

It is a water based gel:

https://en.wikipedia.org/wiki/Medical_ultrasound#Producing_a_sound_wave

https://amzn.to/39J5ydA

It is similar to Nuro-Gel but somehow different, as it is just water based, and if it is drying there are no left overs that crumble.

It is not that expensive and can be ordered at E-Bay or Amazon without any problems, the more you buy the cheaper it is per liter.

It is also quite save as you can get it in a version that is mucous friendly.

May also a good starting substance to get somebody into sploshing.

Cleanup is easy, just rinse with water and you are done.

But be warned, it is extremely slippery, you slide everywhere ;-)

Have fun

Mister K.