Project for small underwater pressure and temperature sensing feasible?

[Paul_B]

@hasep

Perhaps. I just feel like a phone would be easier / cheaper since I already have a phone, and it means less coding on the Arduino side of things

Bad guess.

@hasep

but of course please correct me if I'm wrong

You want to store the data and process later? Definitely store it locally, and you don't have to manage a program on the phone which would make it difficult.

You may however need to include a RTC for a time reference.