Sure thing!

So, the hard thing about conducting an "experiment" in a natural laboratory (i.e., outside - in the real world!) is that there are so many different variables that:
(1) we can't control, and
(2) might not even be able to keep track of.

I think your hypothesis is definitely testable, but I would make sure to be VERY exact in your methodology. Methodology is the way that your experiment is designed and conducted , and it is something that you CAN control. So, although you might not be able to control the ocean, the wind, or the sand, you can control HOW you make your measurements.

Here is my advice:
(1) pick a time of day that you can visit every day (within a 15 minute window should be okay, but if you wanted to be very precise, you should take your measurement at the EXACT same time, every day).
(2) pick a spot to measure from. Measure ONLY at that spot. Take a picture of yourself there so you remember EXACTLY where it is (but also so that you can put it on your science fair poster!).
(3) use the EXACT SAME measuring tool every time. So, if you use a measuring tape, use the SAME one every time. Same thing for a ruler, or whatever you are using to make your measurements.
(4) take your field notebook with you to record your raw data right there, in the field! Trust me - you'll want to make sure you write things down then and there (otherwise, you might forget!)

I hope this has been helpful. Your science project sounds like it is shaping up to be very cool!