Yes try the rechargeable ones that you use in digital cameras, they have twice the charging power of the usual ones. I cant remember exactely what their power is at the moment.
Also if the garmin has a light fuction on it, make sure its turned down or off all together, that will also save power, but at the end of the day these things do love to eat the power! but its cheaper with rechargable ones, keep a spare set charged so when the working set runs out, just replace with fully charged set. Of course the alternative is to hook it into the boats 12v power, thats far better! oh course you said a bike!
maybe its possible to run it off the power supply for your lights on the bike?