I have a plastic boat that has a fairly small hole in the hull. I’ve heard that epoxy is a good idea with plastic kayaks but on the boat, I only have access to one side of the hole. I can see the hole from the outside, but on the inside of the boat there’s an extra plastic mold that completely blocks you from seeing the hole. Is an epoxy still possible if I can’t get to both sides? If it’s not, what are my best options here? Any advice would be helpful. 

You could try the epoxy from just the one side and see if it works, you don't really have anything to lose. Another more labor intensive option would be to separate the hulls so you can get to both sides. It usually involves pulling off the rub rail, drilling out the rivets, and prying it apart. One last option would be something I just saw advertised last night on my ig feed. It was a kayak repair kit but could work for what you have. Basically you heat the cracked area softening it and then push a strengthening mat material into it and then add more plastic over that with a filler rod. I don't for the life of me remember the name of it though. 

I have fixed quite a few holes in kayaks.  This is a good approximation of the process I've used that was shown to me by a kayak fishing buddy.

 https://www.instructables.com/id/Fixing-a-plastic-polyethylene-kayak-with-a-hole-/

I had access to the hull of my kayak, but it wasn't as though I had a hatch right above the hole.  I really don't think I'd put a patch on a sit on top kayak on the inside at any rate.  I used a 5 gallon bucket I got from Home Depot as the patch material the first time I patched it and continued to cut pieces from it for the successive ones.  I didn't use a torch, I used a heat gun I'd bought to melt wax on spit shined boots in the military. A lot of manufacturers will send you a patch piece if you ask for it, and I've kept the piece I cut out for a hatch on my pedal drive.  Aesthetically, your kayak is going to look worse.  My last kayak was red, with basically what looked like a road cone for the rear keel.  It got some looks, and not the type that suggested that they were impressed.  More like I'm pretty sure that guy drives a '87 Toyota pickup with a camper shell that never fit quite right, no muffler, and an inspection sticker he made on his cousin's color printer kind of look.  The patches held for a really long time.  The kayak was never completely dry on the inside like my current kayaks are, but I didn't have a noticeable amount more water coming in until I started putting patch on top of patch.

8 hours ago, Johnny Mills said:

I have a plastic boat that has a fairly small hole in the hull. I’ve heard that epoxy is a good idea with plastic kayaks but on the boat, I only have access to one side of the hole. I can see the hole from the outside, but on the inside of the boat there’s an extra plastic mold that completely blocks you from seeing the hole. Is an epoxy still possible if I can’t get to both sides? If it’s not, what are my best options here? Any advice would be helpful. 

G-Flex epoxy from West marine is made for plastics. I have used it on a couple of repairs with good success. Here is a hilarious video showing the strength of the repair.

 

FM

The best way to repair plastic is to weld it.  Some plastics don't adhere to glues or epoxies well.  Others do okay if you're using the right kind of adhesive.  It depends on what you're working with.  Though, to properly weld plastic, you need to know what kind of plastic you are trying to repair first, and use the same type of plastic (same color if you can find it), so they'll adhere well together.   That can be a tad difficult sometimes.  I've had to go as far as to burn it and smell the burnt plastic to tell what kind it is.  Not fun.  There are websites out there that can give you a list of instructions on how to eliminate various plastics types to narrow down what kind you're working with, if you can't find it through more obvious means.  

 

Besides heat, sometimes the best way to weld plastic is with a solvent, such as acetone.  You mix up the acetone (or whatever solvent you're using) and hardened plastic bits that you wish to use as the patch, and let it dissolve, sometimes for several hours.  Then you can make a nice paste (assuming you don't let it soak too long).  Then just fill in the holes/cracks, and let the solvent evaporate.  It might take a day or two for it to fully harden, depending on how thick the patch is.  If it's thick enough, you might need to poke holes in the skinned over parts to let the solvent evaporate out deep inside, and then refill the holes.  Or better yet, apply the patch in multiple layers, letting each one dry fully before applying the next.  It'll melt the plastic you're trying to repair a bit, giving you a solid, one-piece weld, instead of two pieced held by glue.  Then you just have to sand it down smooth.  It's kind of the same concept as PVC cement, which isn't actually a cement, glue, or epoxy.  It's just a solvent with bits of PVC already dissolved into it.  

 

A lot of glues we're all familiar with employ this basic concept.  Rubber cement is just rubber (or laytex) and solvent.  Super glue is just acrylic plastic and solvent. Standard white school glue is just polyvinyl acetate and water (a solvent).