You don't have a steamer of any kind?
You can just use a bamboo steamer. They're cheap and very useful. You can then cook tons of Asian dishes.
I mean, you could probably simmer them (pasteles, which are the Caribbean equivalent of tamales are simmered), but it's not quite the same, and you'd have to secure them as much as we do pasteles. Tamales don't get tied quite as tight as pasteles.
I say buy the steamer. It's a kitchen tool everyone should have.