I am neither a bacteriologist nor an ichthyologist. However, it is my understanding that tilapia is so popular for food for a few reasons - they grow rapidly, tolerate heavy stocking, tolerate less-than-perfect water conditions, and will thrive on purely vegetarian diets. This last point is important because it makes them cheaper to grow and less likely to have heavy metal contamination. They're popular with consumers because the commonly used hybrids have a pleasing white flesh, nice size fillets, a very mild flavor, and low costs. They are sometimes raised in polycultures where the tilapia clean up the messes and the other species help keep the tilapia population in check but that is not the common scenario for commercial fish farming.
I am doubtful that salmonella in the water will actually contaminate the fish living there. However, I imagine the fish themselves will be covered with infected water that then gets on the fillets in their less-than-sterile processing environment. Freezing doesn't kill salmonella. So, yes, a frozen tilapia fillet could be coated in salmonella-laden ice that then gets spread around the kitchen or directly into your gut if the fish isn't sufficiently cooked (at least 10 or 15 minutes at 140 F).