"I didn't do as well as I'd like during my undergrad
degree, and I don't have a contract
lined up. Will a Master's help?"
Short answer? No.
Firms will always give weight to undergraduate results, and frankly I don't imagine that even a PhD will provide sufficient distraction. Every candidate completes undergraduate studies, and firms like the ease of comparison.
There are exceptions, of course. If grades (and finances) aren't an issue and you have a burning passion to study further in Maritime Law or criminology (or anything else), why not? You can always make it relevant during interviews, by focusing on the research/procedural/firm-relevant stuff you learnt. I have a master's in a fairly obscure branch of literature, gained before I decided to switch to law. When asked, I explain how my previous studies had led me to want to study further in the field, and how I was lucky enough to get a scholarship to help with the costs.
But (and it's a big but), further study is expensive. Plus it's a year (or so) of your life, with all the usual stresses of academic life and study. And the continued uncertainty of not knowing what you'll do afterwards.
I hope to write about this in detail soon, but if you're in the situation described above - no training contract, an OK degree, not sure how to proceed - work experience is usually a better option than further study.