Brilliant!!!!
EDIT: As for strategies, assuming the Marth has the basics of smash down, I would think tilts are gonna be your best bet (Arash said it earlier, but that was during the whole "ROY>MARTH" mess). The only attack of Bowser's that I can ever imagine consistently outspacing some of Marth's moves is your D-Tilt. The sad part is, it would only be outspacing moves like his neutral A, not anything like his fsmash (this is assuming you're not already in a combo =P). U-Tilt would work for some early over-the-top KOs (unless you're on Dreamland 64), and F-tilt (angled up, as Arash suggested) can be handy in interrupting aerials (if they're spaced incorrectly, otherwise, GL).
In the air... no, just stay out of the air unless you think you've got a combo working (even then, Marth's fair is rediculously fast, be careful).
Depending on skill (and if the Marth isn't camping), I guess you COULD hope for a shield-grab on a missed L-cancel, but that really loses weight when the Marth SHDFairs or simply jumps away without fast-falling his first fair. In reality, Marth would only have to do a cross-up nair to prevent sheild-grabbing, Bowser really has nothing on Marth.
Using Fortress out of your shield is an absolute must, but it won't guarantee anything. Against a really good Marth (especially one good at spacing), even that seems hopeless.
Bottom line, if the Marth is any good at spacing, you should really try another character. Your best bets are using your tilts and fortress to max, but you'll still need to be quite a bit better than your opponent to consistently win this matchup.