Saltwater shouldn't be poured outside. If it's not diluted it will kill off almost all vegetation in that location if poured there regularly. It shouldn't go in storm drains - those are for rain water, not saltwater. It is illegal to dump water into storm drains.
Since I own a maintenance company, here are some options I use depending on what clients have available:
Gravel vac waste: I always siphon into a bucket. Always goes into a toilet - if there is lots of sediment I will scoop the bottom of the bucket into a waste bag. In the case of commercial locations where a toilet may be nowhere near then I pour off the water into a sink, and scoop the sediment into a waste bag.
Draining clear water from the tank: If I put the gravel vac buckets into the toilet then I'll put a full 5-gallon bucket of clear water down the toilet afterwards to flush it really well. Bathtubs are a decent choice for the bulk of the clear water because they are low to the ground, and will drain faster. Make sure no sediment goes down a bathtub or sink drain. For larger tanks I use a hose to go wherever the draining happens - sometimes it is not an option, and I just pack buckets, sometimes up and down a flight of stairs. It is what it is. Easier to do when you're getting paid for it though.
I carry a Quiet One 4000 pump hooked up to a 25' hose, and it's good for draining or refilling.
If there is very low carpeting (commercial carpet), or hard floors then a rolling platform dolly works very well. You can get them at Princess Auto - they go on sale 50% off so watch for those. There are some that go up to 600 lbs I think, but good luck pushing that around! I use these in commerical locations for mixing tubs and move the tubs from water stations to the tanks where I then pump out of the tubs into the tank sump.
I'm sure if I think long enough I'll come up with more... haha