Get a real Pleziasaur from Lock Ness, Scotland to just bite a stander by off at the rib cage and squirt real blood on them. The suspense could be built up by an actor portraying the late Marlon Perkings from Mutual of Omaha's Wild Kindom.
Put a guy in a scuba suit and a rubber godzulla costume to jump up out of the water. A prolog by Jaques Costeo.
It could be a pro fisherman pulls out a big one.
It could be a big marrionette with the lights only on the creature, the puppeteers on a scaffold painted flat black and wearing ninja suits. There could be a song from Kermit the swamp years and the monster doing a duet.
A civil war style turtle submarine could be under the water to do puppets from underneath. The monster could be a big black trash bag with styrofoam ball eyes if you are on a budget. To be scarry don't make the monster voices too squeeky.
Steal the Jaws animatronic from Universal Studios Tours.
Or, I like the light idea. It is actually mostly out of the water already but the light comes on and it moves only slightly for the effect.
Or there is this big bubbling in the water first and something really big pops up. No one seems to know where the back hoe went.
Or, it is actually a big clear tube and is sucked up like the vacuum transport at the bank drive through. Put a sign on it noting not to pay any attention to the plexiglas tube for full effect.
Or, just have bubbles in the water and tell them to run! This is either a swimming pool blower for a spa, an air hose off of a compressed air tank and PVC with holes in it to spread out the bubbles or a scuba diver with days of bean eating competitions. Earlier shows could actually have fire on the water.
If it is a wooded trail next to a lake, it could be a long piece of pipe a boom if you wiil, with counterweights actually on the woods side of the path, pulling up a cable with some prelude to look toward the water like bubbles and underwater lights.
If it is an open field and a wide open lake, maybe it goes up with big garage door springs on a metal frame and goes back down with pnuematics.
Maybe it comes up on the path like Jaws? The mechanisms are on the bank of the pond and pull toward the patrons.
I think a guy farting and then jumping up with a black trash bag is the scariest.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.