As much as Seraphine wants to thoroughly enjoy the moment as it is, she still wants to return Eric's embrace after all.
There must be a reason why he has to merge with the lake. It probably has something to do with how she's able to restore the magic it once lost. The Magician of the Tower is the one who's supposed to do it, but it appears like the Crown Prince has taken the job of confirming it. He was most likely trying to sense the wasteland monster's presence, too. Becoming one with this body of water is the fastest way to do all of that.
Seraphine can't help but be greatly impressed, though. From one magic trick after another, Eric really keeps leaving her breathless.
'Milady, are you hurt anywhere? Are you sure you're fine…?" he chimed in again, still feeling more worried than relieved.
"I know you wouldn't drown, but there's still that wasteland monster around-"
"Wait, you knew I wouldn't drown…?" she cut him off, taken aback.