Arduino Underwater ROV

Ok so I am building an underwater ROV sonar prototype. Which is going to be just my arduino, a sonar sensor, and my computer.

Here is what I need to Know
-Which is better underwater for finding things within a distance of 1cm to about a few feet, ultrasound sensor, or a sonar sensor? Which is more accurate underwater?
-Where can I buy the sensor?
-Where can I learn how to program the sensor because I am very new to arduino and I am going a little advanced for my skill level right now.
-If possible, I need an answer quickly, I have to have something built by Oct 10th

Thank you so much in advance, this is for an interview and I am a high school student.

Higher frequency is better resolution. Lower frequency is better range. You'll need to do some testing to find the right trade-off between those two, what's available, and your requirements. Since you don't really have time for any of that, I would try to stick with whatever commercial fish finders use.

I've seen tutorials somewhere for talking to a Humminbird.

-If possible, I need an answer quickly, I have to have something built by Oct 10th

Your project depends on sensors that may/may not exist and if you can find the sensors, they may/may not work the way you want.

I remember something about fish finders using the same NMEA format at GPS to report depth. I can't seem to find it now, but I researched hacking the Humminbird on my fishing boat once and decided it was WAY too complicated to mess with and would have definitely been more Pi than Arduino.