Could have accumulated pre storage. Just an FYI. Seen water build up in the gallons from bad vent. Draining gas tank for storage best idea.
Vent tip The vent image shows it to be plumbed with a high point above opening so any water splashed in will fall out of line. Or there are...
Vent Your vent line for your gas tank is plumbed incorrectly. Look up proper installation.