That looks horrible. But your boat wouldn't have sunk today, tomorrow or in 20 years. You won't find pictures of boats that sunk due to blisters, you just won't. I'm all for spot fixing large blisters or those with the financial means to pay for costly correction.... Otherwise, for most people, it's a waste of time and energy on a 30-40 year old boat to strip, dry out, and apply controversial barrier coats (many failures). The old diesel engine will fail long before the boat sinks from blisters. We'll just have to agree to disagree as to the importance of chasing this 'cosmetic' problem. Here is a good read:
Is the Repair of Blistered Bottoms Mandatory? Based on the foregoing discussion, the obvious conclusion in most cases is negative. If the blisters cannot be shown to be causing significant damage, then repair is certainly not mandatory, despite the many horror stories you may hear from people trying to sell you a costly repair job. If the blisters are large and numerous, it would be wise to seek unbiased, professional advice before you proceed. Bear in mind that blister repair jobs are now big business for boat yards, so that taking advice from yard managers may not be a good idea.
Yes, blisters are unsightly and may cause a resale problem. These are all factors you must weigh, in addition to the very high cost, when deciding to repair or not. Further, you should also be aware that the number of failed blister repair jobs that surveyor's find is very high. No one's ever going to know why blister repairs fail because no one is going to spend the money to find out. That there are so many should also play a role in your decision to repair or not. And even though the repairer may give a warranty on the repairs, you'd best get it in writing and read the fine print. Then make sure the repairer is likely to be around years later to honor that warranty if it becomes necessary.-----David Pascoe