Impossible to play underwater

[motegrey]

Hey guys.

I’m playing on iMac and had no problems until the first underwater sequence. With great effort I managed to reach the shuttle, but here comes one more sequence. This time it’s so bad my iMac freezes and all I can do is turn it off manually. I tried changing all settings to the minimum, but seems like nothing has changed. The game is downloaded from Steam. Here's what it looks like. Excuse my poor frame rate, this was shot using the quicktime player.

Thanks.