Agreed.
Updated.
Anything left besides deciding between CR 6 or CR 7?
Could we say it has vermin-type intelligence rather than being completely mindless (i.e. Int 0 rather than Int —) and give it skills & feats?
At the moment it just has the default skill of Swim +15 from zero skill ranks +8 for having a swim speed and +7 for its strength.
If we go for the "Int 0" option, it would have 13 skill ranks from its 13 HD. How about 0 in Swim (+7 Str & +8 Swim speed), 3 in Climb (+7 Str), 5 in Disguise (-2 Cha, with say a +10 racial bonus to imitate a Pool) and 5 in Listen (-1 Wis), or possible Spot, which would give it the following:
Skills: Climb +10, Disguise +13, Listen +4, Swim +15.
As for Challenge Rating, I'd back CR 6. That's what a 13 HD gelatinous cube would get through advancement, and a Living Pool may be slightly stronger and have a bit of DR, but it also lacks the Cube's Paralysis attack, which makes a big difference.