Cement based fillers: For concrete type pools, repairing drummy areas and (blow) holes etc: (must be suitable for water immersion).
Sealants: For moving joints, cracks.
If painting over sealants use Urethane, NOT silicone-based sealants.
Leaking concrete: (Inc Hydrostatic Pressure)
Rusted steel work:
Cracks in concrete: (Small cracks non-moving) Good hardware shops.
For larger non-moving cracks – see grout, above.