OK, I'll chime in, but I'm not sure I'm the "expert"! (But thanks for the kudos, Stephen!)
Generally, I really try hard to avoid humid environments for any sort of coating - either paint or varnish. It tends to make the finish blush (gets a kind of 'fog' in the top of the finish) and if bad enough may actually trap moisture in the wood that prevents proper adhesion, and the finish may later lift in hot weather or when exposed to direct sunlight. I recommend you either find a controlled environment (air conditioned) or wait until later in the summer when the humidity drops... here in Phila that's usually late August, but YMMV.
Having said that, the recommendation of Epifanes is a good one - it's a very nice product and I've used it before. However, in the case of my own table I used a relatively inexpensive but good quality product - Minwax Helmsman Spar Urethane. I wanted a spar urethane for this job because it better copes with the temperature and humidity extremes found on a boat, as well as offering a tough and scuff resistant finish. Another good one is McCloskey Man O'War Spar Marine Varnish, though I think you'll find that brand takes quite a while longer than the Minwax to dry to a hard finish.
I also chose to use a semi-gloss finish on my table, since the very glossy look doesn't appeal to me as much as a nice soft glow does. However, if you plan to use this finish, you must never apply multiple coats of semi-gloss... you will end up with a dull and cloudy finish. Instead, apply all of your first coats using gloss varnish and save the last coat for your satin or semi-gloss vanish - you'll end up with a nice clear finish and the color of your wood will shine through. (Glossy varnishes don't have any of the 'dulling' elements like semi-gloss or satin finishes, so they go on clear, but multiple coats of semi-gloss or satin will end up applying multiple 'dull' layers and will therefore look cloudy and yellowed.)
As far as sanding between coats, I'm afraid if you want a quality job you're stuck with that one! However, there are a few things that can really minimize how much sanding you have to do...
First, sand the wood as smooth as you can prior to your first application - this will save oodles of work later in the process. Use a 150 or 180-grit paper for most woods. Don't be tempted to use anything higher or you'll clog the wood's pores and prevent good adhesion of the finish.
Second, right before you're ready to apply your first coat, vacuum the product thoroughly with a strong shop vac, using a
clean brush attachment (wash it first if you have to - it might have dirt, grease or oils on it). Get all the dust out of the wood's pores, since your varnish-coated brush will just bring all that rubbish up out of those pores and onto the surface - and leave nasty bumps that you'll have to sand off later on!
Finally, just before you lay the brush to the wood, use a
tack-rag (aka tack-cloth), and wipe down every part of the product, paying particular attention to the corners and crevices. Fold and turn the cloth along the way to expose new, tacky surfaces as you go along. Lightly rub the surface but do not rub hard or the tacky stuff from the rag will end up on the surface and make your finish slower to dry. This tack-rag process will remove all the last little bits of rubbish, dust, lint and other stuff that will spoil your otherwise glass-smooth finish.
Apply your first coat, then after it's dry use a clog-resistant (aka "open coat") 320-grit sandpaper (the grey or white stuff) and use
light pressure... remember, you're just trying to get off the high spots, not remove what you just laid down! Especially avoid sanding corners or edges, as the finish there is the thinnest and you'll expose raw wood
very easily. Only sand the edges if they have visible roughness.
Once you've finished sanding, then get out that vacuum and tack-rag again and continue this process between each coat.
When applying any finish, absolutely scrupulous cleanliness is truly the key!
When you are applying the varnish, try to avoid a lot of back-and-forth brushing... just brush in one direction, in line with the wood's grain where possible. Brush slowly and smoothly, almost like you're 'petting' the surface with the brush, and use long, flowing strokes. Also, try to brush back into your work - that is, brush back into where you've already varnished - this will help prevent 'pulls' in the finish.
Lastly, never, never, never apply any finish in direct sunlight - it will bubble for sure, particularly when dealing with wood. So try to do it in the shade, and under a cover (to prevent falling debris), and shoot for a ambient temperature between 70-78 degrees, if you can.
BTW, I use foam brushes to apply my finishes - I know purists will cringe, but they really are better than bristle brushes, and you're not tempted to re-use them and then impart previously-collected debris into your finish.
