Knick a lava shader from somewhere, change "surfaceparm lava" to "surfaceparm water", then texture the visible surfaces your volcanic swimming pool with it, and the invisible surfaces with the nodraw shader. Here's the modified lava shader from Sokolov:
textures/[yourmapname]/lava_enterable
{
qer_editorimage textures/sokolov/lava1.tga
qer_trans .5
q3map_lightimage textures/sokolov/lava1.tga
q3map_globaltexture
surfaceparm noimpact
surfaceparm water
surfaceparm nolightmap
surfaceparm nobuildsurface
q3map_surfacelight 500
q3map_tessSize 64
cull disable
deformVertexes wave 200 sin 0 1 .5 .5
{
map textures/sokolov/lava1c.tga
rgbGen const ( 0.6 0.6 0.6 )
blendfunc add
tcMod turb .3 .2 1 .05
tcMod scroll .01 .02
}
{
map textures/sokolov/lava1.tga
rgbGen const ( 0.9 0.9 0.9 )
blendfunc add
tcMod turb .2 .1 1 .05
tcMod scale .5 .5
tcMod scroll .02 .01
}
}
Put it in tremulous/base/map-yourmapname.pk3/scripts/yourmapname.shader, and remember to put your map name into your shaderlist.txt else radiant won't find it.