I don't know how to answer, hon. I know a lot of artists in your position. It's why I don't do art as a job- your reputation and how you deliver art is a huge part on if you get bread and butter on your table. Water colours aren't what's holding you back though. There's a LOT of people out there that love the medium. I recently got a set so I could learn :) It's a very delicate medium . Last year at my stall, a water colour artists sold prints of her work and they did very well- people loved seeing non digital medium cause there was just so much of it out there. Another of my friends works on MDF with water colour and does some very rustic feeling art with the tan finish- it makes the white pop awesomely. It's
What you need to understand though, is the market at conventions, and that's not something I can teach you myself, cause every convention is different. What I've noticed in conventions in Auckland, New Zealand is this:-
Armageddon (like Comicon)- Unless you do pre-orders or have eftpos (Apparently eftpos isn't popular in America, but everyone uses it in NZ), people don't usually want to spend more than $20 on one stall purchase. The exception is if they scream when they see your stuff. People that do well have a range between $1 approx to $20 items. Anything over $20 can sell, but in Auckland, people will generally think about it unless you can talk it up (Like comics). I see a lot of badges, sticker sets, mini prints, small comics, and commissions for $10 or less (commissions being sketch stuff- not coloured) and $20 for big comics, A4 / A3 prints. Don't bother with originals here- you wont sell them without having a reputation first- underpriced or not. They may as well be display only. IT's better to do on the spot commissions if you want to sell originals. Stall cost $350 + gst for 3 days (4 if you include preview day) if you can get a stall spot. Last year they went super fast.
Overload (a comic only based convention- use to primarily target Manga audience, but now western styles too)- Overload REALLY loves fanart, I've sold OC art there, but most people will skip over you if they can't scream at their fandom, or be attracted to your OCs for some other more generic reason. It's sad, but that's the culture it has become ^^; And that's cool! These people generally love bright colour, and cute. They don't mind paying a bit more money though for stuff cause it's a more niche market and they get to know you better than Armageddon, but in contrast, they'll run straight past you if they don't interest you.... like LITERALLY run. They come in cosplay in hordes and they want to devour their favs and save up all year for it! You MUST have bright colours to attract people to your stall. You're not allowed to sell crafts unless you buy a craft table though. You can sell originals, but don't make it worth over $100 at this convention. ($50 per table (fits 2 artists easy / $100 for a craft table)
Chromacon (A fine illustration convention targeting high end talent)- This convention started during my hiatus, so I've never actually sold at one, but people at this convention like large gorgeous works of art they can put on the wall. They're willing to spend over $100 for an original that they like, and they scoff at fanart. Comics are considered if they're beautiful comics. Your art has to be a base standard to even apply for a spot (which is intimidating but fair enough).Stall cost- I don't know for certain. I think I remember someone say it was around $150 for a stall. I don't know for certain though.
*This is all generic help of course. There are some exceptional artists that break the mold on their first convention, but as a running trend, this is what I've seen. Other may see different things to me :)
Again- these are conventions in Auckland, but you can see 3 VERY different target audiences even thought they're all art conventions. This is why I strongly suggest going to one first and researching :) Your local one will be very different.
I hope I was helpful for future attempts even if it's tight to do it right now :( I wish you luck in attending one in the future, and I'm sorry that money is super tight right now. I don't have an answer for your situation. I wish I did :(