Overhead or underground?

When you want to realize a powerline crossing of a wide river or a strait, you can realize it either by a span or by an underwater cable. Under which condition is a span more favorable than an underwater cable?

If the banks of the waterway are mountainous, than until a width of up to 5 kilometres an overhead crossing may be the best choice as no tall pylons are required, even when a large clearance for navigation is required.

Interesting question: how much more expensive is such a span than building the line in normal terrain? Or would it be even cheaper as you do not have to erect so many pylons?

When the banks of the waterway are flater, large towers for carrying the span over the waterway are required. Their height depends on span length, required clearance, height of basement over waterway level and strength of conductor ropes. Unfortunately conductor ropes with high strength have worse conductivity as they contain mainly of steel, which has a higher resistance than aluminium or copper.

It should be noted that the powerline crossing of Yangtze River at Jiangyin consists of 2 346.5 metres tall pylons in a distance of 2303 metres. The clearance of this crossing is 55 metres. As the conductors are fixed in 2 levels on the pylons, the conductor sag is approximately 250 metres at this span!
One may ask, if there would be an underwater cable more sensitive.

Interestingly at least one overhead crossing was replaced by a submarine cable. It was the span over Messina Strait with 3646 metres length carried by 2 232 metres tall towers and a clearance of 70 metres. For this span a conductor with high strength consisting nearly completely of steel was required, which allowed only 300 MVA transmission capacity - too low for nowaday's purposes ( unfortunately I have no data about the maximum transmission capacity of Yangtze River crossing at Jiangyin)

On the other hand for the second powerline ( the first one is an underwater HVDC link) to Zhoushan island in China two 370 metres tall towers for a span with 2700 metres length were built and for a similiar span between Java and Bali even taller towers are proposed.

So, there is an interesting question: when should such a crossing be realized overhead and when underground?