Okay, well... I've examined the code, and assuming this description is accurate:
Basically I deliberately did a straight play though to get a high RWC without triggering the dress removal or doing any naughty sex stuff with her.
I went swimming, did the biggest splash, I then hid her bikini top, swam naked with her again (skipped the remove dress flag), Cream blew Sonic, and viola - the 'get naked' flag triggered.
All the other stats were 10+, RWC was around 800 and happiness was around 100.
...I don't see how this condition could have occurred to begin with.
You went swimming, did the biggest splash, then hid her bikini top. So far so good. Going through those events with an RWC of 800 would satisfy the check at the end where you get to keep her top ("Hide the top and claim you can't find it" only succeeds if RWC>400, your Morality is -7 or less, or you've already gotten her to remove her dress), which adds bikini_top to your inventory.
It's the next part that doesn't make sense. The next time you summon her to the back yard and suggest swimming, the code checks to see if you have the bikini top. If you do, the next check is to see if get_naked is set, which changes the menu when she says she doesn't have anything else to swim in -- if get_naked=False, then your choices are "give it back" or "we can just swim without our suits".
When you choose "we can just swim without our suits", the
next check after that requires that your RWC be 500 or higher --
and that you must either have fondled Cream, or went down on her. If you don't have either of those two achievements, she rejects your suggestion and marches back into the house.
So... I can't see any execution path where you could have reached the "swim naked" scene without having at
least fondled her once. And even if you did, she doesn't give you a blowjob in the pool unless you have already achieved at least one of "Got oral from Cream", "Went Down on Cream", or "Vaginal sex with Cream" already.
...are you
sure you didn't have the "went down on cream" achievement set when you went swimming with her the second time? Maybe from mucking about with the cheat menu, or starting from a saved game that had it set?