There is probably a way to blend the two sounds together like morph a goo sound into a crawling sound so it sounds like crawling underwater in goo.
Wet one of those extra thick/absorbent cleaning cloths or a towel. Squish away. Itmight sound a bit wet though...
Make a mush with toilet paper and water. You can easily vary the wetness.
For underwater sounds, cover a make with a condom and record yourself squeezing water from a sports bottle/water pistol/super soaker...or like Dom Lawrence mentioned, playing with a sponge also gives great results...
A bit vague on what exactly you want - is it big/small/human/animal etc etc but try cold pasta and sauce, squeezed and mushed up by hand for the goo. SLowing it down, some short delay/rvb all help to thicken the sound. As for underwater, try mic in a condom and squeeze out spunges underwater and then edit in the results to the picture movements. Then blend with some eq/reverb/spatial plugs to help the underwater feel.
Have you tried vocoding one sound with the other??
Those are great ideas, I especially love the pasta sauce. No experimenting has been done except this question.