So, I am planning on making a box out of acrylic and I would like it to be watertight. I assume the best bet would be to use a watertight caulking or something along the joint edges, but does anyone have other options that may work?
Thanks.
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If you are using acrylic glue, you can make it water tight just by making your joints correctly. Acrylic solvent glue will literally melt the two surfaces together. You just have to make sure that there is a good, even fit between the parts so that the glue wicks into the entire joined surface.
Another option is to use a gel-type acrylic glue which will do the same thing but is thicker so that it stays in place instead of wicking. This is useful when you don’t have perfectly even surfaces to join.
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You may also want to take into consideration the size of the container and the amount of water it will hold. As the weight of the water will put additional stress on the joints/welds, this could be somewhat countered by the thickness of the acrylic used. As thicker acrylic will allow a larger surface area to weld and create increased strength at the joints. Would be good to hear how you get on and what you ended up using. Hope this helps.
Good luck with it
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If you can bend reliably, you can make a U-shaped piece that gets rid of two bottom joints at the cost of making the remaining fabrication/joining a little more finicky if you want it pretty. (Also opens up design possibilities by letting the box only touch its supporting surface at the ends)
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Could make a some T slot on the edges and glue together but like @damsk said it would depend on how much pressure will be pushing out
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Do some research on diy acrylic fish tanks!! The acrylic cement that I used recently (I can’t remember the name for the life of me) is very thin and applied with a syringe to the edge. Worked very well! The only thing about that is that it works on capillary action and when you cut with a laser (from what i understand) its slightly concave (convex? I donno. I was homeschooled ok?).
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That would probably be weld-on aka sci-grip formula 4 or formula 16. You can still melt lasered edges, just need to hold it by hand a little longer before setting it down. The formula 16 is a bit thicker, so would recommend that for the inside of the lasered edge.
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Weld on! Yes! Thank you, I was trying to remember that. 
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Concave: 
Convex: 
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Thanks for the feedback everyone I’ll look into the Weld On.
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But beware, bending introduces something called anticlastic curvature, which makes the edge of the bent sheet tilted and much more difficult to glue, particularly if you want it to be watertight. Here is a figure showing anticlastic curvature. Perhaps that’s what you had in mind when you said “more finicky.”
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My Glowforge forum Word of the Day! Thanks.
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But doesn’t the sense of concave vs convex there depend on which side of the curve you are looking at?
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Highs or lows, ups or downs, apex or nadir, two sides to every argument. 
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Thanks for the tip - I’m texting my older brother about it. Like 15 years ago he was very into making fish tanks with plexiglass, by which I mean he made 3 and they all leaked. Then he declared fish tank making to be impossible. Im not saying he was a terrible fish tank maker, but he has the blood of Duran and Duran on his hands, two innocent goldfish gone too soon.
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You enter a CAVE and go INTO a mountain. ConCAVE goes in.
The lack of an antonym for the word cave is VEXing, so you’re all like “I’m OUT!” ConVEX goes out.
(just gotta remember the first part)
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But doesn’t the sense of concave vs convex there depend on which side of the curve you are looking at?
What is the sound of one hand clapping? When you know this, you will know in which direction the curve points…
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Groan!
- Rich
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His hands are convex! Oh, great, my GlowForge hasn’t even arrived yet, and I have now achieved enlightenment…
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