How can I check whether the data already exists in combobox list?

The ComboBox class has a FindStringExact() method that will do the trick for you, like this:

Dim resultIndex As Integer = -1

resultIndex = cmbTypeYacht.FindStringExact(cmbTypeYacht.Text)

If resultIndex > -1 Then
    ' Found text, do something here
    MessageBox.Show("Found It")
Else
    ' Did not find text, do something here
    MessageBox.Show("Did Not Find It")
End If

You can also just loop through the list as well, like this:

Dim i As Integer = 0
For i = 0 To cmbTypeYacht.Items.Count - 1
    If cmbTypeYacht.Items.Contains(cmbTypeYacht.Text) Then
        MessageBox.Show("Found It")
        Exit For
    End If
Next

I'm working in Excel 2013 and there is no FindStringExact or .Items.Contains so, neither of those are valid. There is also no need to iterate the list. It is very simple actually. Given a userform "MyUserForm" and a combobox "MyComboBox",

If MyUserForm.MyComboBox.ListIndex >= 0 Then
    MsgBox "Item is in the list"
Else
    MsgBox "Item is NOT in the list"
End If

Explanation: If selected item is not in the list, .ListIndex returns -1.