A teenager who jumped into the Whanganui River last week got stuck in the mud with water up to his neck.
Senior Sergeant Andrew McDonald said the 14-year-old was "lucky" the tide was going out at the time and he had landed near one of the bridge pillars.
"The kid jumped off the bridge into the river and got stuck in the mud," he said.
"He was lucky that the tide was going out and that it wasn't another foot deeper because we could have been looking at a tragic accident. He's also lucky that he didn't break his ankles."
The teenager jumped off the Aramoho Railway Bridge and was able to pull his feet out of the silt by holding on to the bridge pillar. Mr McDonald said the drop would have been "a good 10-plus metres".
He warned people diving or swimming around Wanganui to always check the water they were heading into.
"While jumping off the bridges isn't illegal in Wanganui, we do discourage it for that very reason, because logs and all kinds of things could be present just under the water.
"The floods have obviously changed the riverbed, but also with the floods has come a heck of a lot of driftwood and wood down on the bridges. You just don't know what you could be jumping on.
"You just have to look on YouTube for hundreds of examples of incidents where people have jumped off bridges into murky water, only for it to end with some serious injuries."
He warned the beaches would also be changed, and said to "listen to the lifeguards" as summer approaches.