Can a Seafaring race attack the land from a sea or lake region?
Yes, you can attack these new adjacent regions after conquering the seas or lake. The Seafaring rules state that the seas and lake are considered empty regions for you:


  Seafaring
  
  As long as your Seafaring race is active, you may conquer
  the Seas and the Lake, considering them as 3 empty Regions. You keep
  these Regions even once you go into Decline, and continue scoring for
  them for as long as you have tokens there. Only Seafaring races may
  occupy the Seas and the Lake


Since they are just regions, the rules for subsequent conquests apply:


  The active player can repeat this process to conquer as many new
  Regions as he wishes during his turn, provided he has enough Race
  tokens left to accomplish these successive conquests. Each of the
  newly conquered Regions must be adjacent to (i.e. sharing a border
  with) a Region already occupied by his active Race tokens, unless
  permitted otherwise by his Race and Special Power combo.


Go forth and conquer!