Tritium is toxic in the sense of carcinogenicity and induced mutations if inhaled or if tritiated substances are ingested. Obviously it can't injure organisms at a distance. It is insidious because if you are not specifically looking for it, you won't know you have been contaminated. But it certainly is not the most dangerous etc. etc.
Any water soluble radioactive source is dangerous because contaminated water is an easy and pervasive gateway into the body.