MO diagram of Zundel cation
Strictly speaking, drawing a MO diagram is only possible for two entities.
That being said, I will attempt to show you the MOs and order them according to their energy.

At PBE0-D3/def2-TZVPD, using NWChem 6.6, I found the following minimum geometry (vacuum):



Figure 1: Minimum geometry found for Zundel cation ($\ce{[H2O...H...OH2]^{+1}}$) with natural atomic charges, angles (in degrees) and distances (in angstrom)

MOs 1 and 2 correspond to $\ce{1s^2}$ core electrons on oxygen and orbitals 3-10 give us the states for the 8 remaining valence electron pairs.
MO 11 is the lowest unoccupied molecular orbital.



Figure 2: Molecular orbital diagram for structure from Figure 1 at PBE0-D3/def2-TZVPD

Since the structure has no symmetry, no symmetry was assigned to any MO and the diagram above has simple labels only.
Energies (in hartree) can be found further below.



Below are images of each MO:



Figure 3: MO #3 (cutoff 0.04)



Figure 4: MO #4 (cutoff 0.04)



Figure 5: MO #5 (cutoff 0.04)



Figure 6: MO #6 (cutoff 0.04)



Figure 7: MO #7 (cutoff 0.04)



Figure 8: MO #8 (cutoff 0.04)



Figure 9: MO #9 (cutoff 0.04)



Figure 10: MO #10 (cutoff 0.04)



Figure 11: MO #11 (cutoff 0.4)



As a side note, we may obtain a finer view of the MOs by representing them as linear combinations of natural bonding orbitals (NBOs):

CMO: NBO Analysis of Canonical Molecular Orbitals

Leading (&gt; 5%) NBO Contributions to Molecular Orbitals
======================================================
 MO   3 (occ): orbital energy =  -1.356442 a.u.
               0.426*[  5]: BD ( 1) O 4- H 7
               0.400*[  1]: BD ( 1) O 1- H 2
              -0.399*[  2]: BD ( 1) O 1- H 3
               0.396*[  4]: BD ( 1) O 4- H 6
               0.396*[  3]: BD ( 1) O 4- H 5
               0.336*[  9]: LP ( 2) O 1(lp)
 MO   4 (occ): orbital energy =  -1.327547 a.u.
               0.444*[  1]: BD ( 1) O 1- H 2
              -0.440*[  2]: BD ( 1) O 1- H 3
              -0.437*[  3]: BD ( 1) O 4- H 5
              -0.434*[  4]: BD ( 1) O 4- H 6
               0.280*[  9]: LP ( 2) O 1(lp)
              -0.272*[  5]: BD ( 1) O 4- H 7
 MO   5 (occ): orbital energy =  -0.859519 a.u.
              -0.606*[  5]: BD ( 1) O 4- H 7
              -0.502*[  9]: LP ( 2) O 1(lp)
               0.369*[  3]: BD ( 1) O 4- H 5
               0.359*[  1]: BD ( 1) O 1- H 2
 MO   6 (occ): orbital energy =  -0.839700 a.u.
              -0.562*[  4]: BD ( 1) O 4- H 6
               0.511*[  2]: BD ( 1) O 1- H 3
               0.471*[  3]: BD ( 1) O 4- H 5
               0.425*[  1]: BD ( 1) O 1- H 2
 MO   7 (occ): orbital energy =  -0.838909 a.u.
               0.533*[  2]: BD ( 1) O 1- H 3
               0.514*[  1]: BD ( 1) O 1- H 2
               0.481*[  4]: BD ( 1) O 4- H 6
              -0.469*[  3]: BD ( 1) O 4- H 5
 MO   8 (occ): orbital energy =  -0.696009 a.u.
              -0.549*[  9]: LP ( 2) O 1(lp)
               0.502*[  5]: BD ( 1) O 4- H 7
               0.302*[ 10]: LP ( 1) O 4(lp)
              -0.301*[  8]: LP ( 1) O 1(lp)
              -0.253*[  3]: BD ( 1) O 4- H 5
               0.244*[  1]: BD ( 1) O 1- H 2
              -0.227*[  4]: BD ( 1) O 4- H 6
 MO   9 (occ): orbital energy =  -0.636884 a.u.
               0.692*[ 10]: LP ( 1) O 4(lp)
               0.687*[  8]: LP ( 1) O 1(lp)
 MO  10 (occ): orbital energy =  -0.623778 a.u.
              -0.620*[  8]: LP ( 1) O 1(lp)
               0.614*[ 10]: LP ( 1) O 4(lp)
               0.344*[  9]: LP ( 2) O 1(lp)
              -0.319*[  5]: BD ( 1) O 4- H 7
 MO  11 (vir): orbital energy =  -0.201773 a.u.
               0.263*[121]: BD*( 1) O 1- H 2*
               0.262*[123]: BD*( 1) O 4- H 5*
              -0.257*[122]: BD*( 1) O 1- H 3*
               0.257*[124]: BD*( 1) O 4- H 6*
              -0.232*[113]: RY*( 1) H 7(ry*)




Figure 12: Atomic numbering for the NBO Analysis of Canonical Molecular Orbitals above