We have never had a case of waterymouth in 16 years (fingers crossed). I understand that it is more a problem of lambing large numbers indoors, than lambing smaller flocks, especially outside. Please think very carefully whether you can justify the need to use antibiotics routinely where you don't have a known problem - that is the way to create antibiotic-resistant bugs. For navels, 10% iodine (not 5%), especially if dipped not sprayed, is completely adequate for normal lambing.
Pay close attention to hygiene, wearing gloves as discussed above, and using a disinfectant in the lambing shed, to prevent infections rather than using routine antibiotics 'just in case'.