I was also thinking that if you have multiple coastal cities that all have harbors, then if one is besieged it can't be taken by starvation, only by directly assault, unless the attacker gains naval superiority.  This would represent supplies coming in by sea.

A similar concept about providing incentives to build cities near water was discussed in this forum.

https://forums.elementalgame.com/378551

While less than an overwhelming response, it seems like there is at least some agreement that providing benefits for being near the coast makes sense.  The issue I see now is that the "easiest" way that everyone seems to gravitate towards - improving food gathering - has less of an impact now that food is a global resource.  Having something such as "fishing" tiles would still provide benefits to being near the water, but would not necassarily mean that cities near the water would be more likely to grow to large size than inland cities under the global food resource dynamic.

Perhaps then allowing things like Harbors (and/or other building types only available on water tiles) to increase prestige, and therefore city growth, makes the most sense.

I would also like to reiterate that in my opinion building near coasts "should" provide the greatest benefit, but that building near a river should have some modest benefit versus building completely landlocked cities.

Hard to tell without the graphics engine being in.... I would think it's a bug though - Unless salt-water makes Channelers melt, they should be able to walk on the beach... heck they should be able to walk on water too, if they have the right spells.....