The most common reason for an insect not drowning is a result of surface tension.
The high surface tension of the water allows the insect to remain on the surface or float and not drown.
Not sure what insect you're asking about but an example of insect that does not drown or is hard to drown in water are water striders.
Water striders are small insects that are adapted for life on top of still water, using surface tension to their advantage so they can “walk on water.”
The attraction between water molecules creates tension and a very delicate membrane.
Water striders walk on this membrane.
Common insects that can survive water include.
Water striders (Gerridae)
Giant water bugs (toe biters)
Fishing spiders.
Mosquitoes.
Dragonflies.
Insects can walk on water because of a phenomenon called surface tension.
This is essentially a property of water (or any liquid) which allows it to resist an external force.
Other aquatic insects can remain under water for long periods due to high concentrations of hemoglobin in their hemolymph circulating freely within their body.
Hemoglobin bonds strongly to oxygen molecules.