You have answered your own questions. Mangroves and estuaries are not only nursery and feeding grounds for fish but also meeting ground of freshwater and seawater fish resulting new breeds more adapted to these environments. With the inward recession of mangroves and estuaries thee fish with all the above enumerated functions will move inward in proportion specific ecosystem services available for them. Of course, if the mangroves are completely destroyed, mangrove fisheries will be no more there unless same or similar ecosystem is created for them.

Well as we all know mangrove are an extremely vulnerable ecosystem pertaining to their location. A minor sea level rise could inundate a huge chunk of these ecosystems. However, I would like to suggest that not only fisheries but one major group of reptiles which would be affected are the salt water crocodile e.g. at Bhitarkanika National Park, Odisha. There would be a threat of extinction to these animals. At the same time not to mention the regional livelihood around these ecosystems would also be very harshly affected, however they would have time to adapt but what about these poor animals. Along with it destruction of mangrove ecosystems would eradicate a natural barrier to incursions of sea water and high velocity winds putting the nearby communities on the forefront to face these climate vagaries.

http://www.fao.org/docrep/012/i0994e/i0994e03.pdf