Your fears are very understandable. There are poisonous fish and jellyfish in Qatar waters. However, if you stay by the sandy beaches the chances of coming across any of these are very low. Exercise caution - jellyfish are seasonal and not a year-round presence, and if you see them in the water - stay out. If you are camping on the rocky beach make sure you and your family members wear protective footwear - this is usually enough to avoid spiky stonefish (which is very very rare, by the way). However, most beaches in Qatar are sandy so no such thing may be necessary. I think the waters in Qatar are generally very safe. I have never taken any precautions and nothing ever happened. I have come across jellyfish in the water but they are very easy to avoid. And I haven't heard any cases of people being seriously injured or killed by any sea creatures. I am sure you will enjoy the beaches here :-)
Ferrin,
Your fears are very understandable. There are poisonous fish and jellyfish in Qatar waters. However, if you stay by the sandy beaches the chances of coming across any of these are very low. Exercise caution - jellyfish are seasonal and not a year-round presence, and if you see them in the water - stay out. If you are camping on the rocky beach make sure you and your family members wear protective footwear - this is usually enough to avoid spiky stonefish (which is very very rare, by the way). However, most beaches in Qatar are sandy so no such thing may be necessary. I think the waters in Qatar are generally very safe. I have never taken any precautions and nothing ever happened. I have come across jellyfish in the water but they are very easy to avoid. And I haven't heard any cases of people being seriously injured or killed by any sea creatures. I am sure you will enjoy the beaches here :-)