The first of several public forums on the threat of rising sea levels will be held this afternoon in Lewes.   The Delaware Sea  Level Rise Advisory Committee will present displays and information about what an increased ocean level means for Delaware communities.   The committee is developing the state's Sea Level Rise Adaption plan, which will be implemented by a number of state agencies, businesses and residents.  Tonight's meeting will be held in two sessions, at 4:30 and 6:30 p.m. at Cape Henlopen High School.