*A giant snapping turtle gains a +12 racial bonus on Hide checks when in swampy terrain. Further, a giant snapping turtle can lie in the water with only its eyes and nostrils showing, gaining a +10 cover bonus on Hide checks.
That works out as Hide -10* (+2 in swamps, +12 submerged)
OK, let's try some reverse-engineering.
In 1E AD&D, the Giant Snapper has a surprise of 1-4 on d6 and the Crocodile has a surprise of 1-3 on d6.
In 2E AD&D the Giant Snapper has -3 surprise while the Crocodile has -2.
So in both systems the Giant Snapper is about 1/6th
better at surprising than a Crocodile.
A crocodile has Hide +7* (+11 in water, +21 when "lying in water"). To be better than that a giant Snapper needs better values than that.
(By the way, the SRD Croc is missing a skill point somewhere. Its skills add up to 5 ranks while its HD give it 6.)
I suppose we could increase the racial bonus to Hide or the cover bonus for lying in water.
We could also reassign some of the skill ranks.
Come to think of it, a Giant Crocodile's Hide +1* might be a better comparison.
So, how about swapping the Snapper's Endurance for Alertness, then giving it the same +5 Listen and Spot as the SRD Giant Crocodile and putting the rest of its skill points in Hide:
Skills: Hide -1* (+11 in swamps, +21 motionless), Spot +5, Listen +5, Swim +19
Feats: Alertness, Improved Critical (bite), Improved Initiative, Weapon Focus (bite)
*A giant snapping turtle gains a +12 racial bonus on Hide checks when in swampy terrain. Further, a giant snapping turtle can lie motionless in the water, gaining a +10 cover bonus on Hide checks.