As you kiss against the side of the pool you realize this is going to be a little difficult, but you have a better idea - one that occured to you the day Lydia first transformed into a mermaid and you first brought her down to the pool for a swim.
"How the jacuzzi?" you suggest.
"Way ahead of you" said Lydia. "I turned it on before I dove in after you."
Now that she mentioned it, you could hear the water roiling in the jacuzzi. Up till that point you'd either been underwater or otherwise distracted.
You relunctantly release your embrace and the two of pull yourselves onto the side of the pool with the aid of the pool stairs. As you get to the jacuzzi, it occurs to you that you're still wearing your shirt. You'd only removed your pants when your tail formed; now your shirt is of course soaked and you discard.
As you slip into the pool you realize Lydia still hasn't removed her skirt or blouse since she got her tail. She enters the pool and joins you with a kiss. Her soaked blouse clings tightly to her body. You start unbuttoning her blouse as the two of you continue to kiss and are greeted by a surprise. Lydia's breasts have grown with her transformation, as they have in the past, but this time they are still skin rather than spandex.
As you finish the unbuttoning and cast the blouse aside, Lydia removes her skirt and you can see the full difference from her previous transformations: the spandex skin is from the waist down, like yours. You transformed wearing swim trunks, but Lydia had always wore a one-piece before so the spandex covered her torso and arms as well. That meant...
"You got a bikini!" you say, a bit surprised.
Lydia had always prefered more one-piece suits. She had an athletic body with a good figure thanks to her years as a swimmer, but she wasn't the kind to show it off.
"I told you I wanted to surprise you with a new swimsuit. Just something special between the two of us."
You gentley caressed her breasts and kiss her. She responds by wrapping her spandex mer-tail around yours. The feeling is once again electric. Your foreplay continues for a short time, but you feel yourself rising within. At last your penis slips out of its sheath and Lydia lowers herself onto you. As the warm water roils around the two of you, you make love for the first time as merman and mermaid. It's such an amazing feeling you regret a little that it will end. At least, you think it will end, but with the vagueness of Lydia's accidental wish you're still not sure what will happen.