While we're still waiting for the full Environmental-Social Impact Assessment (ESIA) for the Nicaragua Canal to be released, the government of Nicaragua released the Executive Summary of the ESIA yesterday. It can be viewed here. The consulting firm that put the ESIA together is optimistic that the impacts can be mitigated (and even offset to a degree that the overall impact is positive on both the environment and human communities), but the cautionary tone of their conclusions is telling:
[The Project] is fraught with risks. If the Project is not constructed in accordance with international good practice and the proposed mitigation measures are not properly implemented; or if the Project's business case is not realized and the predicted longer term indirect and induced benefits from the Project do not occur; or if the construction of the canal is not completed, Nicaragua may be worse off than doing nothing.
In summary, the Project does offer potential benefits to the environment and people of Nicaragua, but only if its business case is robust, the financing to complete construction is secure, and the Project is constructed and operated to international standards (i.e., recommended mitigation measures are fully implemented).
In other words, it might be possible to construct and operate the canal in a way that provides net benefits to the Nicaraguan people and their environment, but it's not going to be easy.