Section 28. This chapter shall not legalize any structure, filling or other occupation or encroachment, made or done without authority, upon the waters in or the land under any great pond prior to May ninth, eighteen hundred and eighty-eight, or in or upon the Connecticut river, below high water mark, prior to April thirtieth, eighteen hundred and ninety-one, or as waiving or impairing any rights or remedies of the commonwealth or of any person relative thereto.