(1) Your question is not clear. You must have specify the type of solvent. It should be "Alkyl halide are less soluble in water although they are polar."
Alkyl halides are polar molecules and they are held by dipole-dipole attraction. The molecules of water are held together by H-bonds.

Since the new forces of attraction between water and alkyl halide molecules are weaker than forces of attraction already existing between alkyl halide-alkyl halide molecules and water-water molecules, alkyl halides cannot form hydrogen bond with water therefore, alkyl halides are less soluble in water.

While in organic solvent new forces of attraction act between alkyl halide and solvents and are of same strength. Hence the alkyl halides are more soluble in organic solvents.

(2) There is a large electronegativity difference between Cl and C which make the C-Cl bond polar and Cl can easily removed as leaving group in ethyl chloride.
Same C-Cl bond also exist in chlorobenzene but have different characteristics. In chlorobenzene, the electron pair of chlorine atom is in conjugation with π electrons of benzene ring. Thus, chlorobenzene is a resonance hybrid of the following structures:

 
The contributing structures  III, IV and V  indicate that  C-Cl  bond has partial double character.As a result, the  C-Cl  bond in chlorobenzene is shorter and hence, stronger.Thus, cleavage of  C-Cl  bond in benzene becomes difficult. Therefore chlorine cannot be easily replaced in chlorobenzene.