Ironman contest.Thats one tough ass sport! as for your question.,Test Prop is a ED or EOD shot,so once a week will do nothing for you.You should consider a long ester Test like Cypronate or Ethanate where you can take a once a week shot.
With a low dose say 200mg week,you shouldnt expect much water gain,very little.but you can take a AI like Adex to minimize that side effect.Good Luck in your Contest brother!