Slushie... need help

OK I need some help. So monday I decided to go a little off roading and mudding with a buddy of mine (passgener). SO I see this mud puddle and it being my 1st off road experience really I said you think I can make it? He was liek if you go fast enough. So when I was heading around corner to hit mud puddle still contemplating wether I was gonna go through with it. SO i asked again you really think I can make it. he was liek its your truck. I then thought you know he is right screw a warranty lol. So I went in and got stuck. Well water and mud was up to bottom of where door opens. So I finally get pulle dout and water is comming out of frame. Finally water stops comming out so I drive home la di da. Things I learned: Dont go in a puddle unless you know how deep it is. Plus dont do it in a stock vx or street tires, Also make sure you have ample light to see. Yes I know a bit newbish but was good laughs and fun none the less.

Anyways now to question. When I take off or hit the brakes I hear water slushing around somewhere on passnger side. Seems like the length of it. Also @ one point I heard it in back one was length of truck other was width by gas tank. So my question is anyone know where I can start looking because here its wednesday and hanvet found problem. I looked under neath to see if I could possibly trouble shoot. But being not very car oriented or mechanically inclined its like I am looking for a needle in a haystack.

Any and all suggestions will be welcome. Also once friend sends me pictures I will try and post them even tho some may laugh as they have prolly been stuck worse. thanks again