Does someone know a shader that produces the swirling water effect as in old A4 A5 and A6 levels? I know the water reflection shaders look much better, but I'm sort of missing the old effect, it could still be used for swamps or lava or the like.

It's basicall some sine calculations based on time
something like

tex.x += cos(tex.x * Pi * time);
tex.y += sin(tex.x * Pi * time);
color = tex2D(texture0, tex.xy);

sorry if this is syntactically incorrect since I haven't worked with hlsl for a long time.

You need to offset the actually pixel position based on the current pixel (to get the compress/expand effect) and a time value (so it's moving). Just an idea but it could work like that

if you use world coordinates then it wouldn't even be necessary to keep edges and seams in mind.