Most likely toast. I have a Lowrance on a Bass Raider pond boat and it got soaked during a 3 day rain event but was never dunked. Somehow water got inside the screen unit and I couldn't get the water out or get it to work again. I called Lowrance customer service, told them what the issue was and mailed them the unit. They tested it and replaced it under warranty. I now plan on keeping a zip lock bag over it if it is outside while raining.
Gather up your receipts and serial number and give them a call.
I flipped my yak and that had Lowrance HDS 8. It would not power on so I took it to Cabelas and had them hook hook it up to their display model. It powered right up. Long story short during the flip my battery got tossed around loosening the wiring. Simple rewire and I was running again. Might not hurt to check your connections.