Your new tentacle legs move awkwardly with your boxers still on. After a brief struggle, you rip them off and inspect your new half-octopus form. Other than your new "squid hand" and whatever internal changes allow you to breathe water, all the changes to your body seem to be from your waist down (though you're not sure about your back). A quick test confirms you can swim easier now without your boxers constraining your tentacles, though you're not sure how long it will take for you to get over the strange sensation of your new "legs".
You feel the mermaid wrap her arms around you from behind then wrap her tail around your waist just above where your tentacles join your torso
"You make quite the merman."
"Mermen don't have fishtails like yours?"
"Most do, but some of us mermaids like our men a little...different."
"You could have asked!"
"You chose to rescue me, you wanted to swim with me, you chose to put on the ring, and I know you find me attractive. Are you really having second thoughts?"
"This is all quite a shock and my new form is really strange. I'm just..."
The mermaid slide around from my back to in front of me. You recall the mermaid had worn a top before, but now she is barechested. She kisses you, gently at first then deeply. As you kiss, you feel her tail rubbing you somewhere at the base of your tentacles which arouses you. Breaking the kiss, the mermaid smiles at you seductively.