Yes, you are on the right track. First I suggest using your monthly mortgage statements to get the monthly balance on each loan for each month including the houseboat loan.
You'll total the two home loan balances for each month Jan - Apr (you'll have four amounts). You'll total the home loan balances for house B and the houseboat for each of the last 8 months (you'll have 8 amounts). Add the 12 months and divide by 12 to get your average loan balance. If it is below $750,000 all of your mortgage interest paid is deductible. If not, divide it by 750,000 to get a ratio that you can multiply by your loan interest to determine the deductible amount.
To determine what your loan interest paid was you'll need to divide interest paid on house A by 12 and multiply by 4 to get the amount of personal home mortgage interest. You'll do the same for the houseboat except multiply by 8 for that interest.
When enter the 1098 for home A you will indicate the amount is different from your 1098. You probably don't have a 1098 for the houseboat loan so just enter the interest you calculated for box 1 amount as you will be simulating a form 1098.
You'll enter the remaining interest for house A in the rental property expenses section.
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