Nice bag! It's always satisfying to start out with some fabric and thread and make something truly useful.
For the next time I'd consider cutting out the ends of the triangle, since the space is hard to use. Making it flat on the bottom and flat on the front, basically. It means an extra two panels to sew, but I think it's worth it. Also, great job on fitting the zipper, it looks really good. I'd recommend folding the edges of the fabric back under themselves and sewing again, where you cut out for the zipper. That's if you left enough room to do so. It hides the raw edge of the fabric.
What's your fabric? It looks like cordura to me, but it's hard to say over the internet. I don't think cordura is waterproof enough to bother with sealing. Fabrics like Xpac have a fully waterproof layer laminated inside them, but cordura is just a coating. If I were to use a sealer I would use SeamGrip or another liquid sealer before tape; I haven't taped seams before but I'm unsure of how well it would adhere to the cordura texture.
As for using seam grip, I hear that masking tape helps to make clean lines of sealant. I'll be trying it out sometime this week when my 8oz tube of seam grip shows up in the mail.