You do most things 'on' a boat, including live. You might occasionally say that you travel 'in' a boat, but even here the usual construction is 'travel on' a boat.
You travel on business if you are a businessman, and travel for work if you are a worker. So if you are a manager on a trip to do a deal which sub-contracts some of your work, this is travelling on business. However, if you are an oil worker taking a helicopter flight out to the oil rig, this is travel for work.
Information in the mail is information which has been sent through the postal system. Information on the mail is information about the postal system (for example when deliveries and collections happen on your street, or when a letter is due to arrive.)
'Million' when read as a number is generally treated as an adjective, and so, as with other English adjectives, we use the singular, just as we do with thousand and hundred. The exception is when we split number and subject with 'of' (e.g. millions of dollars) as this is no longer an adjective form.