[udontknowmickey]

I never took formal game theory, but I did take micro econ (which the professor tried to teach us game theory) so here goes: Thoughts in white
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assigning probability to attack by land for X as q, and probability for defense of land for Y as p we have the EV of our attack set as

0.6*p*q+1*p*(1-q)+1*(1-p)*q+0.8*(1-p)*(1-q)

which comes out to 0.6*p*q+p-p*q-p*q+q+0.8-0.8*p-0.8*q+0.8*p*q

Which comes out to: -0.6*p*q+0.2*p+0.2*q+0.8

Now I'm treading in unknown waters. I was going to take a first derivative with respect to p but that doesn't seem to get me anywhere to solving for p in terms of q (rather it seems to give me q to minimize p!) So we take a guess and rearrange it as such:

0.2*p-0.6*q*p+0.2*q+0.8 = 0.2*p+q*(-0.6*p+0.2)+0.8

So thus I feel like I want to render Y's choice irrelevant which means setting -0.6*p+0.2 =0 or p=1/3 resulting in EV = 0.8+.2*1/3=0.866666666

To do the same for figuring out q:
0.2*q-0.6*q*p+0.2*p+0.8=
0.2*q+p(-0.6*q+0.2)+0.8

Setting -0.6*q+0.2 = 0 we get 1/3 for q as well

resulting in EV strategy 0.8666666

Optimal strategy for both is 1/3 land 2/3 sea, resulting in a victory probability of 0.8666666666 (or a failure probability of .1333333333)
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PS, cool graphic