@Chainsmoka My guess is she doesn't have enough strength to dive down, exit the boat and swim up on her own. Going by her late husband's estimated age, she's about 90.
I think Konoe could've taken Ren just out of the boat and let him swim up on his on, and then go back to fetch her grandma, but she probably wanted to ensure he would reach the surface safely.
I agree the situation was a little forced, though.