Being Thai, I'm pretty sure that the resemblance is merely coincidental. The words for "*fish*", and "*swim*" in Thai are:

> fish: ปลา `[pla:]`<BR>
> swim: ว่าย `[wa:i]`

Thai has a word for "*fish*" that was borrowed from Pali/Sanskrit:
> มัจฉา: `[mad-cha:]`

which sounds not very close to the mentioned Slavic word.

I'm more curious in how words like "elephant", "avatar", "immortal", which Thai borrowed from Pali/Sanskrit as เอราวัณ `[e:-ra:-wa:n]` อวตาร `[a-wa-tan]` อมตะ `[a-ma-ta]`, sound like in Slovic. (NOTE: The more common word for elephants, which is not a borrowed word, in Thai is ช้าง `[cha:ng]`.)