Undersea tunnels allow shipping to pass but require higher costs of security while low bridges can cause traffic congestion and higher bridges are expensive.
An undersea tunnel is a tunnel which is partly or wholly constructed under the sea or an estuary. One such advantage would be that a tunnel would still allow shipping to pass. Tunnels require far higher costs of security and construction than bridges. A low bridge would need an opening or swing bridge to allow shipping to pass, which can cause traffic congestion. Higher bridges can also be more expensive than lower ones. 
