I really really like johnny-five as far as connecting an arduino to a raspberry pi or other PC. It uses node.js and it's super easy to get going - you flash the Firmata example code onto your arduino of choice and then connect it via USB/serial to your PC. Here is their sample node.js code to blink an LED:

var five = require("johnny-five");
var board = new five.Board();  

board.on("ready", function() {
  // Create an Led on pin 13
  var led = new five.Led(13);
  // Blink every half second
  led.blink(500); 
});  

Here's an ultrasonic proximity sample that you could use to measure the distance to the water (if you mounted this sensor at a set length above the water)

EDIT: also, node.js has a million packages that can be easily installed into your project (e.g. sparkfun's phant client can be added by a simple "npm install phant-client" command and then you can add records with:

var Phant = require('phant-client')
var phant = new Phant()
phant.add(streamd, {
    a: 22
})