A really good underwater housing and counterweight set for a DSLR is expensive. If you are a professional underwater photographer it can be worth the money. Otherwise, probably not.
There are one-size-fits-many economy housings that have an adequate glass plate to look through, and adequate water tightness. However, they can be a struggle to use.
Improvisations like Zip lock bags are a bad idea at many levels. Leaking, optical distortion, nearly impossible manipulation of controls, incorrect buoyancy adjustment.
You could bring along a compact waterproof camera. Quality won't match what the T3i in a professional housing could do, but it may be decent. The Nikon AW110 is currently running about $230. It has a slight negative buoyancy, and will sink to the bottom if dropped in the water, so get a floating wrist strap for it.
Some limitations and precautions:
Avoid getting any particles of sand, dust, hair etc on the gasket. That can cause leaking.
After exposure to salt water, rinse outside with fresh water and let thoroughly dry before opening the battery compartment.
The water resistance is considered adequate for most snorkeling, but not enough for the depth and duration typical of SCUBA diving.
Warm, moist ambient air in your surface environment gets trapped inside he camera and may condense to fog the lens when you hit the cooler water below the surface. For underwater housings this is averted by inserting silica gel packs in the housing. The compact waterproof camera has no space to do this. You can pre-dry the camera's interior by placing it in a sandwich bag containing silica gel packs with battery compartment open for several hours. Then insert a fully charged battery and carefully close the battery compartment before setting out on your adventure.
Remember, you cannot change batteries underwater, and you want to avoid doing so in a damp environment.
Do not use rice as a drying agent. Its effectiveness is poor. It can shed particles that can get onto the gasket and cause leaking.