A while back we covered the most venomous fish. And even though we are the Fisheries Blog, we still report on other aquatic critters now and then. Given the interest in our list of venomous fish, we wanted to expand to include other things in the ocean that can ruin your day. We are also not requiring that these animals be venomous or poisonous, though many are.

5.Sea Snake

Part of the cobra family of snakes, these widely-distributed sea snakes, or kraits, are very venomous. The good news is they are not aggressive and have very small mouths, so the chances of being bitten are exceedingly small.

4. Cone Snail

As beautiful as the shell may be, avoid these snails at all cost. These snails carry their own venom—conotoxin—and its effects are serious and not fully known.

3. Blue-ringed Octopus

With its distinctive blue circles, this octopus carries tetrodotoxin, which is a venom that stops your breathing and is 1,200 times more toxic than cyanide. Fortunately, the octopus is not aggressive and will avoid you before you can likely avoid it.

2. Box Jelly

This group actually includes several species that can kill you—the Irukandji jellyfish, the sea wasp, and common kingslayer (pictured above). Able to kill humans in under 5 minutes, these jellies are found in the Indo-Pacific region, but with some species distributed elsewhere.

1. Saltwater Crocodile

Surpassing 20 feet in length, the saltwater crocodile is known for being aggressive toward humans. Death counts are not well known because reports are questionable and in much of the saltwater crocodile’s range there is no reporting. Avoid at all costs!

Note: This list has been assembled by The Fisheries Blog and is in no way comprehensive of dangerous sea creatures, nor is this list in any order from least to most dangerous. As with all dangerous animals, use precautions—know what risks might be in the area, obey all postings, and seek medical attention if encountered. Fortunately, many dangerous sea creatures are like most other animals and will avoid you as you avoid them.