If it's from the original series it's probably The Probe, which is the
only one that ended with people floating on the water. No tentacles,
but there is a giant amoeba creature in the probe.
http://en.wikipedia.org/wiki/The_Probe
http://membres.lycos.fr/tmcr/daystar/tol_s2/tol_eps49.shtml
If it's from the revival series it might be Virtual Future. Don't
remember any tentacles there, either.
http://en.wikipedia.org/wiki/Virtual_Future_(episode)
http://www.theouterlimits.com/episodes/season1/118.htm
But it may be neither of these. I haven't seen all of the revival
episodes.
**
Captain Infinity
>>At the end of this episode, you see this guy stranded on some other
>>world. He is floating in water (ocean, sea?), in a boat I think, and
>>below him is this gigantic creature in the water, it has tentacles.
>> Please email me if you have the answer. indexai (at) gmail.com.
>>Thanks.
I think this might be it, "Worlds Apart"
http://en.wikipedia.org/wiki/Worlds_Apart_(The_Outer_Limits)
**
Captain Infinity