Near the Narrows on the Grand Canyon found an iPhone, I think it's a 7, in a waterproof carrying case. We were on the river June 13th-25th, not sure how long it had been there, but it's on airplane mode and locked.
one thought - you should be able to pull up the control center menu by swiping up from the bottom edge of the screen. that'll let you turn off airplane mode. doing so should let the owner use find my iphone to display a message on the phone's screen for you. just a thought ... can't hurt to try. good luck and hope your float in the big ditch was great!