Water ain't great at absorbing gamma mate, not as good as a dense metal. A gamma has a very large mean free path in water, I work with calibrating and characterising tools that measure density using gamma sources. Water is excellent for neuton absorbtion though, which is what makes neutron radiation very harmful to humans too being % water.
Alpha radiation itself is pretty harmless, they use it in industry a lot, even smoke detectors. What is harmful is the alpha emitting radio isotopes which are ingested and decay internally. Ideal for a dirty bomb.