I can almost guarantee that there will be water flowing in March/April, but how grand the flow is will be unpredictable. Typically the biggest water flows will being around May, but in big snow years it can still be massive in early June and up to July. However, I was looking for an excuse to go to Yosemite, and checking the Yosemite Falls webcam, I never really saw it "big" this year. It's always kind of complex in what makes the waterfalls full. They need lots of water, and of course there it comes from melting snow. So you'll need a combination of lots of snow and lots of sun/heat to melt that snow. Some years the waterfalls look great in early May and peter out. I think this year there's been so little snow that it never really got going. There are predictions for lots of snow, so maybe next year will be promising.
Personally I think any time of year is good for Yosemite. The rock generally isn't going anywhere, although it does shed every now and then. I personally like it during the winter with a nice coating of snow. You might not be able to hike the Mist Trail, but you didn't say how much interest you had in hiking.
As for your logistical questions.....
The closest major airport is in Fresno (in fact it's called Fresno-Yosemite International Airport). There may be regional flights to Merced. However, neither are really serviced that well and prices might not be that good. There are a few other ways to get in, but Reno is probably too far away and there's that issue of closed roads. Mammoth has an airport, but again road closure will be an issue. I think most visiting Yosemite would fly out of the Bay Area airports, with Oakland probably being the most convenient to get to Yosemite (via I-238 and I-580). Bass Lake is near the south entrance to Yosemite, so that might take a bit of time to get to.
The major roads (Wawona Road and within Yosemite Valley) probably won't be closed, although there is the slight possibility of chain controls if it's actively snowing. Wawona Road is likely to be clear. That's kind of an odd time of year for Glacier Point Road (to Badger Pass and Glacier Point), which is higher in elevation and gets more snow. The Badger Pass ski season will probably be over so they'll stop plowing until they're ready to plow all the way to Glacier Point. Tioga Road (in Yosemite's high country) tends to open in May, but can open in June.
This has a listing of opening dates for Tioga Road and Glacier Point Road:
http://www.nps.gov/yose/planyourvisit/tiogaopen.htm
If you want to go that time of year, I'd say do it. The crowds may be smaller, and the temps will be cooler. You probably won't have the mosquito issues from late spring or early summer. A lot of the services will be limited though. Also - March next year is when Aramark is supposed to be taking over the concession, so it might get interesting how the handover occurs.
