mandapam to Rameshwaram undersea bridge is possible but second half plan is very tough as Gulf of mannar and palk strait have bio geography constraints to allow big vessals. only smaller ferry can be done. For India to Srilanka they can easily do Kodiyakarai (near AGX) to Point petro in jaffna also. even there is not even talk abt this ancient route restoration which will save plenty of resources than present Tuticorin to columbo.