No such structure shall be built or maintained upon or over the lands of the state or lands underlying the navigable waters of the state so as to in any wise unreasonably obstruct navigation, or the freedom of the use of the navigable waters of the state for commerce and navigation, or for harbor purposes; nor shall any charge be exacted of any vessel, barge, boat or raft, either singly or in fleets, for anchoring, mooring or tying up or remaining on the navigable waters, presumptive or established, either or both of them, alongside of or on the lands of the state abutting thereon or thereunder, not then occupied by the structures and improvements placed thereon or therein pursuant hereto; or in the waters in front of said wharves and other structures so long as the reasonable use of said wharves and other structures occupying the lands of the state, or the aforesaid navigable waters of the state or the use of the approaches to said walls and other structures, or the coaling of vessels using same, is not unreasonably restricted, interfered with or prevented thereby; subject, however, to the further reservation and right of the Legislature or other authority constituted by it for such purpose, to vacate or cause to be vacated and repossess or cause to be repossessed by the state so much of said riparian lands belonging to it, as may be at any time necessary for use by the state or federal government in aid of navigation and commence or for harbor purposes, or to insure the freedom and safety of navigation or the public or the property abutting upon the navigable waters of the state, but not otherwise, and to that end may proceed under its right of eminent domain as to any structures thereon, so that the right of the state and the public may be preserved and insured under Section 24 of Article 1 of the Constitution of Alabama.