Although I will concede that pathogens to which they had developed immunity might prove dangerous to humans and that such microbes may have survived in this previously isolated lake.
I wouldn't be entirely worried about pathogens to which dinos had or had not developed immunity to as we are not reptilian - bar the royals of course

- and thus those pathogens are unlikely to be able to do anything deleterious without mutation. Besides, such microbes are as likely to be anywhere else on the planet as they are down an isolated lake so it's a risk that's been taken time and again I'd think.
Personally I think we will be doing little more than contaminating one of the few pristine environments left on our planet.
I tend to disagree with such sentiments, what matter is the pristine-ness - taken to mean completely free of human presence - of a place if we never go near it? It may as well not exist in that case and hence being pristine is an empty quality, just an ephemeral notion that somewhere over there is something 'pristine'.
Hope is but the first step upon the road to disappointment.