You can buy a single-hulled, 30 year old tanker for around $5 million, empty (still operable, but on the way to the scrapyard). Say it's a smallish tanker, only a million barrels or so. Buy oil today at less than $50 / barrel and hold it til it's back up to $100. I could live on the profit for a while, quite nicely.
Be prepared to spend money on whatever fees are necessary to keep that ship moored or tied up to a dock; then, add costs for the maintenance that it will require and 24/7/365 security.