I think everyone has used a water level at one time or another using a clear hose, I know I have hundreds of times. Here's an idea that some of you already know but I didn't until talking to a mate at work. get a bucket cut a hole in the bottom and attach your hose to it, (i used a 19mm to 13mm poly reducer from bunnings) then fill your bucket and hose with water put the bucket roughly where you want your mark and then take the other end and place a mark there and so on. There you have your level and then you can measure up or down from that at each point to where you want