The App is not designed to have all the functionality of the full site. It is developed to be a scaled down version to check in on the go. I doubt we will add a ton more functionality, as the more buttons and functions we add, the harder it is to use. Even with the current functionality we get a lot of questions and user error (i.e. hitting report instead of reply, etc).
As for the forum subsections, they are under constant review, and we have made several changes the past 6 months or so, with some more on the way. Dividing brackish from freshwater is one of the possible changes, but it is in discussion.
We don't generally make drastic changes, but rather incremental changes, as it can be quite a lot of work to constantly move around thousands of threads to keep the site indexed properly. We weigh very carefully the decision and the need for new or changing sections.