Umm, I'm a licensed structural engineer and this is (kind of) overbuilt. I say (kind of) because I'd frame it differently, but the way that you've drawn it, you don't actually have a lot more wood in there than you need. The bending capacity of frame is proportional to the square of the depth, so you will get a lot farther going with frame members that are deeper than 2x4s. i.e. if you used 2x6s, you could almost remove all of the interior columns: the allowable stress for No. 1 grade doug fir under wet use with permanent loads is 918 psi, if you space your joists at 16" they would be at 931 psi, so if you went with 12" spacing, you'd be good to go. That being said, deflection could be a problem (I calculate it at about 1/16" under that load). For peace of mind I'd probably go with select structural grade 4x6s or, even better, 2x8s.
This isn't really that much load when it comes down to it, because it's distributed across a large area, even being very conservative, I'd say your load might be 220 pounds per square foot (psf), the water alone is 187 psf; this is similar to the loading for a commercial storage (filing) room or library. Most of the heavy industrial structures I work with are designed for 600-1000 psf.
You need to make sure to pay attention to the lumber grade. There is a HUGE difference in the load carrying capacity of the framing depending on the lumber - 'stud' grade lumber is barely over half the capacity of No. 1 grade. I wouldn't even consider building something like this with less than No. 1 grade lumber, select structural would be better. A lot of standard lumber yards don't have anything better than 'stud' grade in 2x4 and sometimes 2x6, but even Lowes or Home Depot usually carry 'No. 1 & Better' for 2x8's or larger.
There are additional reductions in the capacity if you have notches cut in the framing or use incised (pressure treated) lumber. An additional benefit of this is that usually the higher grade stuff is straighter.
-Justin
