yes, many small pockets of methane seep out of lakes around here, the anaerobic conditions in lake bottom mud and peat support the methanogenic bacteria that produce the gas. the problem is not methane being toxic per say, as it is having methane pouring out in great quantities from the sea floor. Without being able to test the water and air around the spill, we don't know how much associated toxic hydrocarbons are also in the mix. No, CO2 is not toxic, on the contrary it supports organic life. Added rapidly to the atmosphere though, it seems to speed up the pace of natural climate change, to the detriment of most forms of life. Fast change is usually harmful.