I don't have twins, but I do have 3 young children that I've taken to the pool and beach numerous times by myself. I strongly urge you to get a lifeguard approved PFD (personal floatation device aka life jacket). They make them for infants and toddlers and certainly have them in your twins' size. Go to a boat/marine place (the store in Castleton has a decent selection) and have the kids try them on. The different brands fit a little different and fit is crucial.
My youngest LOVES the water. I put a PFD on her and she kicks away. I stay close to her while the other play but I don't have to have my hand on her constantly. I still put my 3 year old (well, she'll be 4 next month) in a PFD as well because she can't swim.
Do NOT rely on the arm wings/floaties or the 'boats'. They are fun but are NOT designed to keep kids' heads above water. Many kids drown or come close to it while wearing/sitting in these because it's too tempting to look away for minute to deal with older kids. Also be carefull of the swimsuits with build-in floatation aids. They are not meant for life saving and you still need to have your hands on them because many of the designs can force a young child to lay face down in the water. :-( They are not designed to keep the heads above water... they are designed to be used to aid while learning to swim - with an adult right there.
A suggestion for the PFD... the infant kind (usually in sizes up to 30lbs or 50lbs) have a pillow thingy behind the head. This forces the child to lay on her/his back in the water with head above the water. It is very safe but pretty uncomfortable for the toddler if they like to kick and play. My 1 year old (she'll be 2 in August) is wearing the 30-50lbs child size PFD which looks like a normal adult PFD. It keeps her pretty vertical in the water so she can kick, 'swim' and play around. I also suggest a PFD for your 3 year until she/he can swim.
We have a neighborhood pool and we also spend summers at our lakehouse and in our boats (kayaks, row boats, ski boats, rafts, etc) so we're around water ALOT. You can never be too safe when it comes to kids and water. Having coast guard approved PFD will do wonders to ease your stress level and let you enjoy the time with your children... even if they fall in (or jump in while you turn your back) they'll be OK.